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/llm/context_builder.py

@@ -30,6 +30,15 @@ def _outcome(home_goals: int, away_goals: int, side: str) -> str:
     return "W" if away_goals > home_goals else ("D" if away_goals == home_goals else "L")
 
 
+def _is_stats_available(stats, before) -> bool:
+    """检查统计数据在 cutoff 时间是否已可用。"""
+    if before is None:
+        return True
+    if stats.available_at is None:
+        return True  # 无时间信息时保守处理:允许使用
+    return stats.available_at <= before
+
+
 @dataclass
 class MatchContext:
     match_id: int
@@ -162,7 +171,8 @@ async def stats_slice(header: MatchHeader, *, limit: int = 10, before=None) -> s
                 gf += fm.home_goals if side == "home" else fm.away_goals
                 ga += fm.away_goals if side == "home" else fm.home_goals
                 n += 1
-                if fm.stats:
+                # 只使用 cutoff 之前已可用的统计数据
+                if fm.stats and _is_stats_available(fm.stats, before):
                     if fm.stats.home_shots is not None:
                         shots += fm.stats.home_shots if side == "home" else fm.stats.away_shots
                         sot += fm.stats.home_shots_on_target if side == "home" else fm.stats.away_shots_on_target