feat(ask): Phase 3.5b guardrails — verifier + telemetry + grounding 강화

Phase 3.5a(classifier+refusal gate+grounding) 위에 4개 Item 추가:

Item 0: ask_events telemetry 배선
- AskEvent ORM 모델 + record_ask_event() — ask_events INSERT 완성
- defense_layers에 input_snapshot(query, chunks, answer) 저장
- refused/normal 두 경로 모두 telemetry 호출

Item 3: evidence 간 numeric conflict detection
- 동일 단위 다른 숫자 → weak flag
- "이상/이하/초과/미만" threshold 표현 → skip (FP 방지)

Item 4: fabricated_number normalization 개선
- 단위 접미사 건/원 추가, 범위 표현(10~20%) 양쪽 추출
- bare number 2자리 이상만 (1자리 FP 제거)

Item 1: exaone semantic verifier (판단권 잠금 배선)
- verifier_service.py — 3s timeout, circuit breaker, severity 3단계
- direct_negation만 strong, numeric/intent→medium, 나머지→weak
- verifier strong 단독 refuse 금지 — grounding과 교차 필수
- 6-tier re-gate (4라운드 리뷰 확정)
- grounding strong 2+ OR max_score<0.2 → verifier skip

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

app/services/search_telemetry.py

@@ -25,6 +25,7 @@ from typing import Any
 from sqlalchemy.exc import SQLAlchemyError
 
 from core.database import async_session
+from models.ask_event import AskEvent
 from models.search_failure import SearchFailureLog
 
 logger = logging.getLogger("search_telemetry")
@@ -306,3 +307,47 @@ async def record_search_event(
             failure_reason="low_confidence",
             context=base_ctx,
         )
+
+
+# ─── /ask 전용 telemetry (Phase 3.5b) ─────────────────────
+
+
+async def record_ask_event(
+    query: str,
+    user_id: int | None,
+    completeness: str | None,
+    synthesis_status: str | None,
+    confidence: str | None,
+    refused: bool,
+    classifier_verdict: str | None,
+    max_rerank_score: float,
+    aggregate_score: float,
+    hallucination_flags: list[str],
+    evidence_count: int,
+    citation_count: int,
+    defense_layers: dict[str, Any],
+    total_ms: int,
+) -> None:
+    """ask_events INSERT. background task에서 호출 — 에러 삼킴."""
+    try:
+        async with async_session() as session:
+            row = AskEvent(
+                query=query,
+                user_id=user_id,
+                completeness=completeness,
+                synthesis_status=synthesis_status,
+                confidence=confidence,
+                refused=refused,
+                classifier_verdict=classifier_verdict,
+                max_rerank_score=max_rerank_score,
+                aggregate_score=aggregate_score,
+                hallucination_flags=hallucination_flags,
+                evidence_count=evidence_count,
+                citation_count=citation_count,
+                defense_layers=defense_layers,
+                total_ms=total_ms,
+            )
+            session.add(row)
+            await session.commit()
+    except SQLAlchemyError as exc:
+        logger.warning(f"ask_event insert failed: {exc}")