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>

app/api/study_concepts.py

@@ -15,6 +15,7 @@ from core.auth import get_current_user
 from core.database import get_session
 from models.user import User
 from services.study import concept_curriculum as cc
+from services.study import concept_links as cl
 
 router = APIRouter()
 
@@ -43,6 +44,20 @@ async def get_today_concepts(
     return await cc.today_concepts(session, user.id, topic_id, limit)
 
 
+@router.get("/concepts/weakness-map")
+async def get_weakness_map(
+    user: Annotated[User, Depends(get_current_user)],
+    session: Annotated[AsyncSession, Depends(get_session)],
+    topic_id: int = DEFAULT_TOPIC_ID,
+    limit: int = 12,
+):
+    """개념 약점 지도 — 링크된 기출 정답률로 약점 개념(정답률<60%) 우선(이론↔문제)."""
+    name = await cc._topic_name(session, topic_id)
+    if not name:
+        return {"weak": [], "weak_total": 0, "evaluated_total": 0}
+    return await cl.weakness_map(session, user.id, name, limit)
+
+
 @router.get("/concepts/{doc_id}")
 async def get_concept_detail(
     doc_id: int,
@@ -57,6 +72,17 @@ async def get_concept_detail(
     return detail
 
 
+@router.get("/concepts/{doc_id}/questions")
+async def get_concept_questions(
+    doc_id: int,
+    user: Annotated[User, Depends(get_current_user)],
+    session: Annotated[AsyncSession, Depends(get_session)],
+    limit: int = 20,
+):
+    """개념 관련 기출 + 내 정답률 (이론↔문제 브리지)."""
+    return await cl.related_questions(session, user.id, doc_id, limit)
+
+
 @router.post("/concepts/{doc_id}/read")
 async def post_concept_read(
     doc_id: int,