feat(search): Phase 0.3 검색 실패 자동 로깅

검색 실패 케이스를 자동 수집해 gold dataset 시드로 활용.
wiggly-weaving-puppy 플랜 Phase 0.3 산출물.

자동 수집 트리거 (3가지):
- result_count == 0           → no_result
- confidence < 0.5            → low_confidence
- 60초 내 동일 사용자 재쿼리   → user_reformulated (이전 쿼리 기록)

confidence는 Phase 0.3 휴리스틱 (top score + match_reason).
Phase 2 QueryAnalyzer 도입 후 LLM 기반으로 교체 예정.

구현:
- migrations/015_search_failure_logs.sql: 테이블 + 3개 인덱스
- app/models/search_failure.py: ORM
- app/services/search_telemetry.py: confidence 계산 + recent 트래커 + INSERT
- app/api/search.py: BackgroundTasks로 dispatch (응답 latency 영향 X)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/api/search.py

@@ -2,7 +2,7 @@
 
 from typing import Annotated
 
-from fastapi import APIRouter, Depends, Query
+from fastapi import APIRouter, BackgroundTasks, Depends, Query
 from pydantic import BaseModel
 from sqlalchemy import text
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -11,6 +11,7 @@ from ai.client import AIClient
 from core.auth import get_current_user
 from core.database import get_session
 from models.user import User
+from services.search_telemetry import record_search_event
 
 router = APIRouter()
 
@@ -38,6 +39,7 @@ async def search(
     q: str,
     user: Annotated[User, Depends(get_current_user)],
     session: Annotated[AsyncSession, Depends(get_session)],
+    background_tasks: BackgroundTasks,
     mode: str = Query("hybrid", pattern="^(fts|trgm|vector|hybrid)$"),
     limit: int = Query(20, ge=1, le=100),
 ):
@@ -52,6 +54,9 @@ async def search(
             vector_results = await _search_vector(session, q, limit)
             results = _merge_results(results, vector_results, limit)
 
+    # Phase 0.3: 실패 자동 로깅 (응답 latency에 영향 X — background task)
+    background_tasks.add_task(record_search_event, q, user.id, results, mode)
+
     return SearchResponse(
         results=results,
         total=len(results),