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/models/search_failure.py

@@ -0,0 +1,28 @@
+"""search_failure_logs 테이블 ORM — 검색 실패 자동 수집 (Phase 0.3)"""
+
+from datetime import datetime
+from typing import Any
+
+from sqlalchemy import BigInteger, Boolean, DateTime, Float, ForeignKey, Integer, String, Text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.database import Base
+
+
+class SearchFailureLog(Base):
+    __tablename__ = "search_failure_logs"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
+    query: Mapped[str] = mapped_column(Text, nullable=False)
+    user_id: Mapped[int | None] = mapped_column(
+        BigInteger, ForeignKey("users.id", ondelete="SET NULL")
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=datetime.now, nullable=False
+    )
+    result_count: Mapped[int] = mapped_column(Integer, nullable=False)
+    confidence: Mapped[float | None] = mapped_column(Float)
+    failure_reason: Mapped[str] = mapped_column(String(30), nullable=False)
+    context: Mapped[dict[str, Any] | None] = mapped_column(JSONB)
+    reviewed: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)