feat(study): 비슷한 문제 검색 (PR-5)

study_questions 자동 임베딩(PR-4 bge-m3 1024차원) 기반 cosine 유사도
top-K. 26B 호출 없음, vector search 만. additive UI — 기존 입력·복습
흐름 영향 없음.

백엔드: GET /api/study-questions/{id}/similar?limit=5&topic_only=true
- 자기 자신/soft-deleted/embedding_status!=ready 제외
- topic_only=true (default) 면 같은 study_topic 안에서만
- 응답: items[{id, question_text(80자 truncate), subject, scope, exam_round,
  similarity(1-cosine), attempt_count, last_correct}], source_status, source_id
- 현재 문제 embedding 미생성/실패/stale 시 빈 결과 + source_status 안내
- attempt_count + last_correct batch 조회 (N+1 회피)

프론트:
- 편집 화면(/questions/[qid]/edit): 페이지 로드 시 자동 GET /similar →
  카드 5개. 본문 truncate + subject/scope/exam_round + 유사도 % + attempt
  배지 (정/오답 아이콘). 카드 클릭 시 해당 문제 편집 페이지로 이동.
- 복습 화면(/review): 답 제출 후 "비슷한 문제 보기" 토글 → expand 5개 카드.
  같은 형태. 다음 문제로 cursor 이동 시 자동 닫힘.
- 통합뷰: 변경 없음 (이미 편집 진입점이 시각적 cue 역할).

source_status별 안내 (pending/failed/stale/none): 임베딩이 아직 준비 안 됐을
때 "약 1분 안에 cron 자동 처리" 메시지 노출.

후속 PR 예정: subject/scope 자동 추천(PR-6), 오답노트/통계(PR-7),
AI 풀이 idle batch(PR-8). 현재 PR-5 는 vector search 결과 노출까지만.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/api/study_questions.py

@@ -178,6 +178,28 @@ class AttemptResponse(BaseModel):
     stats: QuestionAttemptStats
 
 
+# ─── PR-5: 비슷한 문제 검색 (embedding cosine) ───
+
+
+class SimilarQuestionItem(BaseModel):
+    id: int
+    study_topic_id: int
+    question_text: str  # 80자 truncate
+    subject: str | None
+    scope: str | None
+    exam_name: str | None
+    exam_round: str | None
+    similarity: float  # 1 - cosine_distance, 1.0=동일, 0.0=무관
+    attempt_count: int
+    last_correct: bool | None
+
+
+class SimilarQuestionsResponse(BaseModel):
+    items: list[SimilarQuestionItem]
+    source_status: str  # 현재 문제의 embedding_status. 'ready' 가 아니면 빈 결과.
+    source_id: int
+
+
 # ─── PR-3: AI 풀이 생성 ───
 
 
@@ -714,6 +736,119 @@ async def submit_attempt(
     )
 
 
+# ─── PR-5: 비슷한 문제 검색 (embedding cosine) ───
+
+
+@router.get(
+    "/study-questions/{question_id}/similar",
+    response_model=SimilarQuestionsResponse,
+)
+async def list_similar_questions(
+    question_id: int,
+    user: Annotated[User, Depends(get_current_user)],
+    session: Annotated[AsyncSession, Depends(get_session)],
+    limit: int = Query(5, ge=1, le=20),
+    topic_only: bool = Query(True, description="True 면 같은 study_topic 안에서만"),
+):
+    """현재 문제의 embedding 기준 cosine 유사도 top-K. 26B 호출 없이 vector search 만.
+
+    제외:
+      - 자기 자신 (id != current)
+      - soft-deleted
+      - embedding_status != 'ready' (대기 중·실패·미생성)
+
+    source_status='ready' 가 아니면 items 빈 배열 반환 (UI 에서 안내).
+    """
+    src = await session.get(StudyQuestion, question_id)
+    src = _verify_question_ownership(src, user)
+
+    if src.embedding_status != "ready" or src.embedding is None:
+        return SimilarQuestionsResponse(
+            items=[], source_status=src.embedding_status, source_id=question_id
+        )
+
+    # cosine 거리 = embedding <=> ref. 유사도 = 1 - distance.
+    distance_expr = StudyQuestion.embedding.cosine_distance(src.embedding)
+    base = (
+        select(StudyQuestion, distance_expr.label("distance"))
+        .where(
+            StudyQuestion.user_id == user.id,
+            StudyQuestion.id != question_id,
+            StudyQuestion.deleted_at.is_(None),
+            StudyQuestion.embedding_status == "ready",
+            StudyQuestion.embedding.is_not(None),
+        )
+        .order_by(distance_expr.asc())
+        .limit(limit)
+    )
+    if topic_only:
+        base = base.where(StudyQuestion.study_topic_id == src.study_topic_id)
+
+    rows = (await session.execute(base)).all()
+    similar_qs = [(r[0], float(r.distance)) for r in rows]
+
+    # attempt_count + last_correct batch (N+1 회피)
+    qids = [q.id for q, _ in similar_qs]
+    attempt_count_map: dict[int, int] = {}
+    last_correct_map: dict[int, bool] = {}
+    if qids:
+        cnt_rows = (
+            await session.execute(
+                select(
+                    StudyQuestionAttempt.study_question_id,
+                    func.count().label("total"),
+                )
+                .where(
+                    StudyQuestionAttempt.user_id == user.id,
+                    StudyQuestionAttempt.study_question_id.in_(qids),
+                )
+                .group_by(StudyQuestionAttempt.study_question_id)
+            )
+        ).all()
+        for r in cnt_rows:
+            attempt_count_map[r.study_question_id] = int(r.total)
+
+        latest_rows = (
+            await session.execute(
+                select(
+                    StudyQuestionAttempt.study_question_id,
+                    StudyQuestionAttempt.is_correct,
+                )
+                .where(
+                    StudyQuestionAttempt.user_id == user.id,
+                    StudyQuestionAttempt.study_question_id.in_(qids),
+                )
+                .order_by(
+                    StudyQuestionAttempt.study_question_id,
+                    StudyQuestionAttempt.answered_at.desc(),
+                )
+                .distinct(StudyQuestionAttempt.study_question_id)
+            )
+        ).all()
+        for r in latest_rows:
+            last_correct_map[r.study_question_id] = bool(r.is_correct)
+
+    def _truncate(text: str, n: int = 80) -> str:
+        return text if len(text) <= n else text[:n].rstrip() + "…"
+
+    items = [
+        SimilarQuestionItem(
+            id=q.id,
+            study_topic_id=q.study_topic_id,
+            question_text=_truncate(q.question_text, 80),
+            subject=q.subject,
+            scope=q.scope,
+            exam_name=q.exam_name,
+            exam_round=q.exam_round,
+            similarity=round(1.0 - dist, 4),
+            attempt_count=attempt_count_map.get(q.id, 0),
+            last_correct=last_correct_map.get(q.id),
+        )
+        for q, dist in similar_qs
+    ]
+    return SimilarQuestionsResponse(items=items, source_status="ready", source_id=question_id)
+
+
 # ─── PR-3: AI 풀이 생성 엔드포인트 ───
 
 # MLX 호출 timeout (초). MLX gate + 26B 추론 평균 ~10s, 안전 마진.

frontend/src/routes/study/topics/[id]/questions/[qid]/edit/+page.svelte

@@ -11,7 +11,7 @@
   import { goto } from '$app/navigation';
   import { api } from '$lib/api';
   import { addToast } from '$lib/stores/toast';
-  import { ArrowLeft, Save, Trash2, AlertCircle, Sparkles } from 'lucide-svelte';
+  import { ArrowLeft, Save, Trash2, AlertCircle, Sparkles, GitCompare, ArrowRight, CheckCircle2, XCircle } from 'lucide-svelte';
   import Button from '$lib/components/ui/Button.svelte';
   import Card from '$lib/components/ui/Card.svelte';
   import Skeleton from '$lib/components/ui/Skeleton.svelte';
@@ -35,6 +35,24 @@
   let is_active = $state(true);
   let stats = $state({ attempt_count: 0, correct_count: 0, wrong_count: 0 });
 
+  // PR-5: 비슷한 문제 검색 결과
+  let similarLoading = $state(false);
+  let similarItems = $state([]);
+  let similarSourceStatus = $state('none');
+
+  async function loadSimilar() {
+    similarLoading = true;
+    try {
+      const res = await api(`/study-questions/${questionId}/similar?limit=5&topic_only=true`);
+      similarItems = res.items ?? [];
+      similarSourceStatus = res.source_status ?? 'none';
+    } catch {
+      similarItems = [];
+    } finally {
+      similarLoading = false;
+    }
+  }
+
   // PR-3: AI 풀이 상태 + 본문 캐시
   let aiStatus = $state('none'); // none | pending | ready | failed | stale
   let aiGeneratedAt = $state(null);
@@ -93,6 +111,8 @@
       aiStatus = q.ai_explanation_status ?? 'none';
       aiGeneratedAt = q.ai_explanation_generated_at;
       aiModel = q.ai_explanation_model;
+      // PR-5: 비슷한 문제 자동 로드 (페이지 로드와 병렬)
+      loadSimilar();
     } catch (err) {
       addToast('error', err.detail || '문제 로딩 실패');
     } finally {
@@ -288,6 +308,69 @@
       {/snippet}
     </Card>
 
+    <!-- PR-5: 비슷한 문제 5개 (embedding cosine, 26B 호출 없음) -->
+    <Card class="mb-3">
+      {#snippet children()}
+        <div class="p-4 flex flex-col gap-3">
+          <div class="flex items-center gap-2">
+            <GitCompare size={16} class="text-accent" />
+            <span class="text-sm font-semibold text-text">비슷한 문제</span>
+            <span class="text-[11px] text-dim">{similarItems.length}건</span>
+          </div>
+
+          {#if similarLoading}
+            <div class="text-xs text-dim">불러오는 중…</div>
+          {:else if similarSourceStatus !== 'ready'}
+            <div class="text-xs text-dim">
+              {#if similarSourceStatus === 'pending'}
+                임베딩 생성 중입니다. 잠시 후 새로고침하면 비슷한 문제가 표시됩니다.
+              {:else if similarSourceStatus === 'failed'}
+                임베딩 생성 실패. 다음 cron 틱(약 1분)에 자동 재시도됩니다.
+              {:else if similarSourceStatus === 'stale'}
+                문제 본문 변경 후 임베딩 재계산 대기 중입니다. 잠시 후 새로고침해주세요.
+              {:else}
+                임베딩 미생성 (none). 약 1분 안에 cron 이 자동 처리합니다.
+              {/if}
+            </div>
+          {:else if similarItems.length === 0}
+            <div class="text-xs text-dim">이 주제에 비슷한 문제가 아직 없습니다.</div>
+          {:else}
+            <ul class="flex flex-col gap-1.5">
+              {#each similarItems as it (it.id)}
+                <li>
+                  <a
+                    href={`/study/topics/${it.study_topic_id}/questions/${it.id}/edit`}
+                    class="flex items-center gap-3 p-2.5 rounded border border-default bg-bg/30 hover:border-accent hover:bg-accent/5 transition-colors"
+                  >
+                    <div class="flex-1 min-w-0">
+                      <div class="text-sm text-text truncate">{it.question_text}</div>
+                      <div class="text-[11px] text-dim mt-0.5 flex items-center gap-2 flex-wrap">
+                        {#if it.subject}<span>{it.subject}</span>{/if}
+                        {#if it.scope}<span>· {it.scope}</span>{/if}
+                        {#if it.exam_round}<span>· {it.exam_round}</span>{/if}
+                        {#if it.attempt_count > 0}
+                          <span class="ml-auto flex items-center gap-1">
+                            {#if it.last_correct === true}
+                              <CheckCircle2 size={11} class="text-success" />
+                            {:else if it.last_correct === false}
+                              <XCircle size={11} class="text-error" />
+                            {/if}
+                            <span>{it.attempt_count}회</span>
+                          </span>
+                        {/if}
+                      </div>
+                    </div>
+                    <span class="text-[11px] text-accent font-medium shrink-0">{Math.round(it.similarity * 100)}%</span>
+                    <ArrowRight size={12} class="text-dim shrink-0" />
+                  </a>
+                </li>
+              {/each}
+            </ul>
+          {/if}
+        </div>
+      {/snippet}
+    </Card>
+
     <div class="flex items-center justify-between gap-2 flex-wrap">
       <div class="flex gap-2">
         <Button href={`/study/topics/${topicId}`} variant="ghost" icon={ArrowLeft}>주제로</Button>

frontend/src/routes/study/topics/[id]/review/+page.svelte

@@ -11,7 +11,7 @@
   import { page } from '$app/stores';
   import { api } from '$lib/api';
   import { addToast } from '$lib/stores/toast';
-  import { ArrowLeft, Play, CheckCircle2, XCircle, RotateCcw, Sparkles, AlertCircle } from 'lucide-svelte';
+  import { ArrowLeft, Play, CheckCircle2, XCircle, RotateCcw, Sparkles, AlertCircle, GitCompare, ArrowRight } from 'lucide-svelte';
   import Button from '$lib/components/ui/Button.svelte';
   import Card from '$lib/components/ui/Card.svelte';
   import Skeleton from '$lib/components/ui/Skeleton.svelte';
@@ -54,6 +54,38 @@
     aiExpl = null;
     aiExplError = null;
     aiExplLoading = false;
+    // PR-5: 비슷한 문제 expand 도 함께 reset
+    simOpen = false;
+    simItems = [];
+    simLoading = false;
+    simSourceStatus = 'none';
+  }
+
+  // PR-5: 비슷한 문제 더 풀기
+  let simOpen = $state(false);
+  let simLoading = $state(false);
+  let simItems = $state([]);
+  let simSourceStatus = $state('none');
+
+  async function toggleSimilar() {
+    if (simOpen) {
+      simOpen = false;
+      return;
+    }
+    simOpen = true;
+    if (simItems.length > 0) return; // 이미 로드됨
+    const q = questions[cursor];
+    if (!q) return;
+    simLoading = true;
+    try {
+      const res = await api(`/study-questions/${q.id}/similar?limit=5&topic_only=true`);
+      simItems = res.items ?? [];
+      simSourceStatus = res.source_status ?? 'none';
+    } catch {
+      simItems = [];
+    } finally {
+      simLoading = false;
+    }
   }
 
   async function loadAiExplanation(regenerate = false) {
@@ -343,6 +375,67 @@
               {/if}
             </div>
 
+            <!-- PR-5: 비슷한 문제 더 풀기 (embedding cosine, 26B 호출 없음) -->
+            <div class="rounded border border-default bg-bg/30 p-3 flex flex-col gap-2">
+              <div class="flex items-center gap-2 flex-wrap">
+                <GitCompare size={14} class="text-accent" />
+                <span class="text-xs font-semibold text-text">비슷한 문제 더 풀기</span>
+                {#if simItems.length > 0}
+                  <span class="text-[10px] text-dim">{simItems.length}건</span>
+                {/if}
+                <span class="ml-auto">
+                  <Button size="sm" variant="ghost" onclick={toggleSimilar} loading={simLoading}>
+                    {simOpen ? '접기' : '비슷한 문제 보기'}
+                  </Button>
+                </span>
+              </div>
+
+              {#if simOpen}
+                {#if simSourceStatus !== 'ready'}
+                  <div class="text-[11px] text-dim">
+                    {#if simSourceStatus === 'pending'}임베딩 생성 중입니다. 잠시 후 다시.
+                    {:else if simSourceStatus === 'failed'}임베딩 생성 실패. 다음 cron 자동 재시도.
+                    {:else if simSourceStatus === 'stale'}본문 변경 후 임베딩 재계산 대기 중.
+                    {:else}임베딩 미생성. 약 1분 안에 cron 처리.
+                    {/if}
+                  </div>
+                {:else if simItems.length === 0}
+                  <div class="text-[11px] text-dim">이 주제에 비슷한 문제가 없습니다.</div>
+                {:else}
+                  <ul class="flex flex-col gap-1.5">
+                    {#each simItems as it (it.id)}
+                      <li>
+                        <a
+                          href={`/study/topics/${it.study_topic_id}/questions/${it.id}/edit`}
+                          class="flex items-center gap-3 p-2 rounded border border-default bg-surface hover:border-accent transition-colors"
+                        >
+                          <div class="flex-1 min-w-0">
+                            <div class="text-xs text-text truncate">{it.question_text}</div>
+                            <div class="text-[10px] text-dim mt-0.5 flex items-center gap-2 flex-wrap">
+                              {#if it.subject}<span>{it.subject}</span>{/if}
+                              {#if it.scope}<span>· {it.scope}</span>{/if}
+                              {#if it.attempt_count > 0}
+                                <span class="ml-auto flex items-center gap-1">
+                                  {#if it.last_correct === true}
+                                    <CheckCircle2 size={10} class="text-success" />
+                                  {:else if it.last_correct === false}
+                                    <XCircle size={10} class="text-error" />
+                                  {/if}
+                                  <span>{it.attempt_count}회</span>
+                                </span>
+                              {/if}
+                            </div>
+                          </div>
+                          <span class="text-[10px] text-accent font-medium shrink-0">{Math.round(it.similarity * 100)}%</span>
+                          <ArrowRight size={11} class="text-dim shrink-0" />
+                        </a>
+                      </li>
+                    {/each}
+                  </ul>
+                {/if}
+              {/if}
+            </div>
+
             <div class="flex justify-end">
               <Button onclick={next}>{cursor + 1 >= questions.length ? '결과 보기' : '다음 문제'}</Button>
             </div>