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/llm/context_builder.py

@@ -328,9 +328,19 @@ async def _load_match(db, match_id: int) -> Match:
 
 
 async def _get_form(db, team_id: int, before, *, limit: int = 5) -> list[Match]:
-    """某队近 N 场(已完赛)。before=None 表示不限制(预测赛前的场景由调用方保证)。"""
+    """某队近 N 场(已完赛)。before=None 表示不限制(预测赛前的场景由调用方保证)。
+
+    必须预加载 stats / home_team / away_team:切片函数会读取这些关系,
+    而 async session 下惰性加载会抛 MissingGreenlet。
+    (models.py 已声明 lazy="selectin",此处显式声明以固化查询意图。)
+    """
     stmt = (
         select(Match)
+        .options(
+            selectinload(Match.stats),
+            selectinload(Match.home_team),
+            selectinload(Match.away_team),
+        )
         .where(Match.match_status == "finished")
         .where(Match.home_goals.is_not(None))
         .where((Match.home_team_id == team_id) | (Match.away_team_id == team_id))
@@ -344,9 +354,13 @@ async def _get_form(db, team_id: int, before, *, limit: int = 5) -> list[Match]:
 
 
 async def _get_h2h(db, home_id: int, away_id: int, before, *, limit: int = 5) -> list[Match]:
-    """两队交锋史。"""
+    """两队交锋史。需预加载 home_team / away_team(切片输出队名)。"""
     stmt = (
         select(Match)
+        .options(
+            selectinload(Match.home_team),
+            selectinload(Match.away_team),
+        )
         .where(Match.match_status == "finished")
         .where(Match.home_goals.is_not(None))
         .where(
@@ -363,9 +377,17 @@ async def _get_h2h(db, home_id: int, away_id: int, before, *, limit: int = 5) ->
 
 
 async def _get_home_away(db, team_id: int, side: str, before, *, limit: int = 10) -> list[Match]:
-    """某队主场/客场近 N 场。side='home' 取主场,'away' 取客场。"""
+    """某队主场/客场近 N 场。side='home' 取主场,'away' 取客场。
+
+    当前只用标量字段,但统一预加载以免后续扩展时踩坑。
+    """
     stmt = (
         select(Match)
+        .options(
+            selectinload(Match.stats),
+            selectinload(Match.home_team),
+            selectinload(Match.away_team),
+        )
         .where(Match.match_status == "finished")
         .where(Match.home_goals.is_not(None))
         .order_by(Match.match_date.desc())