feat(ad-ecpm): trace_id 端到端落库 — 模型/迁移/Schema/Repository/Endpoint(A1-A3+A5)

- AdEcpmRecord 新增 trace_id(String 64, index, nullable)列,与 ad_feed_reward_record 一致
- 迁移 ad_ecpm_trace_id:ADD COLUMN + CREATE INDEX,同时收敛双 head
  (11c44afbea58 selfstat + merge_pages_override_coupon_slot → 单 head)
- EcpmReportIn.trace_id 字段透传客户端本场比价/领券 session ID
- create_ecpm_record 新增 trace_id 参数并落库
- ecpm_report 端点 forward payload.trace_id 到 repository

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

app/api/v1/ad.py

@@ -286,6 +286,7 @@ def ecpm_report(payload: EcpmReportIn, user: CurrentUser, db: DbSession) -> Ecpm
         ad_session_id=payload.ad_session_id,
         adn=payload.adn, slot_id=payload.slot_id,
         feed_scene=payload.feed_scene,
+        trace_id=payload.trace_id,
         app_env=payload.app_env, our_code_id=payload.our_code_id,
     )
     logger.info(