fix(P0): 修复三处静默失效的阻断缺陷

P0-1 backtest 会话生命周期:
- _get_historical_matches 加 selectinload(league/home_team/away_team),
  并在 session 内物化为 BacktestCandidate 纯数据快照,避免 session
  关闭后访问惰性关系抛 MissingGreenlet
- except 收窄并改用 logger.exception 保留堆栈
- 移除未使用的 and_ / League 导入

P0-2 切片函数关系属性 MissingGreenlet:
- models.py 为 Match.league/home_team/away_team/stats 声明 lazy=selectin
- context_builder 的 _get_form/_get_h2h/_get_home_away 显式 selectinload
  (修复 form_slice/h2h_slice 恒定失败,被 fail-open 掩盖的问题)
- repositories.find_by_teams_and_date 加 selectinload(Match.stats)

P0-3 bzzoiro raw 变量泄漏导致血缘错乱:
- normalized_matches 改为携带 (nm, raw) 元组,内层循环解包
- source_event_id / source_record_id 现在取到正确 event id
- 已有比赛补建 stats 时补齐 source/source_event_id/retrieved_at/available_at
- 抽取 _match_key()/_to_date() 统一日期键构造 (P1-6)

src/db/repositories.py

@@ -37,12 +37,17 @@ class MatchRepository:
     async def find_by_teams_and_date(
         self, league_id: int, home_team_id: int, away_team_id: int, date
     ) -> Match | None:
-        """按联赛+主队+客队+日期查找比赛(天级匹配)。"""
+        """按联赛+主队+客队+日期查找比赛(天级匹配)。
+
+        预加载 stats:调用方(understat 回填)会读取 existing.stats,
+        async session 下惰性加载会抛 MissingGreenlet。
+        """
         if hasattr(date, "date"):
             date = date.date()
 
         stmt = (
             select(Match)
+            .options(selectinload(Match.stats))
             .where(Match.league_id == league_id)
             .where(Match.home_team_id == home_team_id)
             .where(Match.away_team_id == away_team_id)