feat(study): 이론↔문제 브리지 (Stage B) — 개념별 정답률·약점 개념 지도

이론공부 B→A→C 의 B. 완성된 문제풀이에 이론 연결(약점 구동).
- 마이그 382 study_concept_links(개념 doc↔기출, FK 없음) + 백필 SQL(임베딩 코사인 top-k=10·threshold 0.62 → 2362링크·284개념·964문항)
- concept_links 서비스(related_questions·weakness_map 롤업) + GET /concepts/{id}/questions·/concepts/weakness-map(라우트 순서=weakness-map 먼저)
- 리더 관련기출 섹션(정답률·문항 stub→문항상세) + 홈 약점개념 위젯
- 적대리뷰 반영: Promise.all 격리(weakness-map 실패→코어 대시보드 블랙아웃 방지)·q.subject null 폴백. 백필=배포 후 트랜잭션 래핑 실행. 문제풀이 무접촉

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/study/+page.svelte

@@ -4,7 +4,7 @@
   import { onMount } from 'svelte';
   import { api } from '$lib/api';
   import { addToast } from '$lib/stores/toast';
-  import { BookOpen, PenLine, GraduationCap, FolderKanban, Layers, Repeat, Flag, Inbox, Activity, CalendarCheck } from 'lucide-svelte';
+  import { BookOpen, PenLine, GraduationCap, FolderKanban, Layers, Repeat, Flag, Inbox, Activity, CalendarCheck, Target } from 'lucide-svelte';
 
   let cardReviewCount = $state(0);
   let questionFlagCount = $state(0);
@@ -12,6 +12,7 @@
   // 오늘의 공부 (이론 홈)
   let curriculum = $state(null);
   let todayConcepts = $state([]);
+  let weakConcepts = $state([]);        // 약점 개념(관련 기출 정답률 낮음)
   let dashLoading = $state(true);
 
   let readPct = $derived(
@@ -28,10 +29,15 @@
       curriculum = cur;
       todayConcepts = today?.concepts ?? [];
     } catch {
-      // 대시보드 실패해도 허브 나머지는 동작 (조용히)
+      // 코어 대시보드 실패해도 허브 나머지는 동작 (조용히)
     } finally {
       dashLoading = false;
     }
+    // 약점 개념 = 비차단(신규 엔드포인트 실패해도 코어 대시보드 블랙아웃 방지)
+    try {
+      const weak = await api('/study/concepts/weakness-map?limit=5');
+      weakConcepts = weak?.weak ?? [];
+    } catch {}
   }
 
   async function markRead(doc) {
@@ -121,6 +127,22 @@
           {/each}
         </ul>
       {/if}
+
+      {#if weakConcepts.length > 0}
+        <div class="mt-4 pt-3 border-t border-default">
+          <div class="text-xs text-dim mb-2 flex items-center gap-1.5">
+            <Target size={13} class="text-error" /> 약점 개념 <span class="text-faint">(관련 기출 정답률 낮음)</span>
+          </div>
+          <div class="flex flex-wrap gap-2">
+            {#each weakConcepts as w (w.doc_id)}
+              <a href="/study/read/{w.doc_id}"
+                class="text-xs rounded-full border border-error/40 bg-error/10 text-error px-3 py-1 hover:bg-error/20 transition-colors">
+                {w.title.replace(/^\d+_/, '')} <span class="font-semibold">{w.accuracy}%</span>
+              </a>
+            {/each}
+          </div>
+        </div>
+      {/if}
     {/if}
   </section>
 

frontend/src/routes/study/read/[docId]/+page.svelte

@@ -13,11 +13,12 @@
   import Button from '$lib/components/ui/Button.svelte';
   import EmptyState from '$lib/components/ui/EmptyState.svelte';
   import Skeleton from '$lib/components/ui/Skeleton.svelte';
-  import { BookOpen, ArrowLeft, Eye, EyeOff, Check, ChevronLeft, ChevronRight } from 'lucide-svelte';
+  import { BookOpen, ArrowLeft, Eye, EyeOff, Check, ChevronLeft, ChevronRight, FileQuestion } from 'lucide-svelte';
 
   let docId = $derived($page.params.docId);
 
   let concept = $state(null);
+  let relatedQ = $state(null);          // 관련 기출(이론↔문제, 비차단)
   let loading = $state(true);
   let notFound = $state(false);
   let mode = $state('read');            // 'read' | 'recall'(떠올리기)
@@ -25,12 +26,17 @@
   let marking = $state(false);
 
   const STAGE_LABEL = { 0: '복습 시작', 1: '복습 1단계', 2: '복습 2단계', 3: '복습 3단계', 4: '학습 완료' };
+  const OUTCOME_MARK = { correct: '○', wrong: '✕', unsure: '?' };
+  const OUTCOME_CLASS = { correct: 'text-success', wrong: 'text-error', unsure: 'text-warning' };
+  const outcomeMark = (o) => OUTCOME_MARK[o] ?? '–';
+  const outcomeClass = (o) => OUTCOME_CLASS[o] ?? 'text-faint';
 
   async function load() {
     const reqId = docId; // in-flight 가드: 백링크 연타 시 stale 응답 무시
     loading = true;
     notFound = false;
     concept = null;
+    relatedQ = null;
     revealed = {};
     mode = 'read';
     try {
@@ -41,9 +47,15 @@
       if (reqId !== docId) return;
       if (e?.status === 404) notFound = true;
       else addToast('error', '개념을 불러오지 못했습니다');
+      return; // 본문 실패 → 관련기출 스킵
     } finally {
       if (reqId === docId) loading = false;
     }
+    // 관련 기출(비차단 — 실패해도 본문 표시엔 영향 없음)
+    try {
+      const rq = await api(`/study/concepts/${reqId}/questions?limit=6`);
+      if (reqId === docId) relatedQ = rq;
+    } catch {}
   }
 
   // $effect 가 마운트 1회 + docId 변경(백링크/이전·다음) 재로드를 모두 커버 (onMount 불필요)
@@ -202,6 +214,29 @@
       </section>
     {/if}
 
+    <!-- 관련 기출 (이론↔문제 브리지) -->
+    {#if relatedQ && relatedQ.linked > 0}
+      <section class="mb-5 rounded-lg border border-default bg-surface p-4">
+        <h2 class="text-sm font-semibold text-text mb-2 flex items-center gap-1.5">
+          <FileQuestion size={15} class="text-accent" /> 관련 기출
+          <span class="ml-1 text-xs font-normal text-dim">
+            {relatedQ.linked}문항{#if relatedQ.accuracy !== null} · 정답률 <span class="{relatedQ.accuracy < 60 ? 'text-error' : 'text-text'} font-medium">{relatedQ.accuracy}%</span>{:else} · 아직 안 풂{/if}
+          </span>
+        </h2>
+        <ul class="space-y-0.5">
+          {#each relatedQ.questions as q (q.id)}
+            <li>
+              <a href="/study/topics/4/questions/{q.id}"
+                class="flex items-center gap-2 text-xs py-1 text-dim hover:text-accent transition-colors">
+                <span class="{outcomeClass(q.last_outcome)} shrink-0 w-4 text-center font-bold">{outcomeMark(q.last_outcome)}</span>
+                <span class="truncate">{q.subject ?? '기출'}{#if q.exam_round} · {q.exam_round}{/if}</span>
+              </a>
+            </li>
+          {/each}
+        </ul>
+      </section>
+    {/if}
+
     <!-- 액션바 -->
     <div class="flex items-center gap-2 border-t border-default pt-4 mt-2">
       {#if concept.prev_id}