feat: P0-02/P0-03/P0-05 数据库时间语义与约束

P0-02: MatchStats 增加 source/source_record_id/retrieved_at/available_at
        context_builder 过滤统计数据时检查 available_at <= cutoff
P0-03: Prediction 增加 match_kickoff_at/prediction_created_at/prediction_cutoff_at
        明确区分比赛时间/预测创建时间/数据截止时间
P0-05: Prediction 增加 status 字段(success/failed/degraded)
        数据库 CHECK 约束

Alembic: 0005_prediction_status_and_stats_provenance

P1-02: injuries as_of 不再截断为 date,保持 datetime 精度

src/data/understat.py

@@ -10,6 +10,7 @@ import json
 import logging
 import random
 import re
+from datetime import datetime, timezone
 
 from sqlalchemy import func, select
 
@@ -136,7 +137,14 @@ class UnderstatSource:
 
             # 回填 xG
             if existing.stats is None and (nm.home_xg is not None or nm.away_xg is not None):
-                existing.stats = MatchStats(match_id=existing.id)
+                now = datetime.now(timezone.utc)
+                existing.stats = MatchStats(
+                    match_id=existing.id,
+                    source="understat",
+                    source_event_id=str(raw.get("id", "")),
+                    retrieved_at=now,
+                    available_at=now,
+                )
                 db.add(existing.stats)
                 await db.flush()
             if existing.stats is not None: