refactor: Sprint 3 - 引入 UnitOfWork + Repository 架构

新增:
- src/db/unit_of_work.py: UnitOfWork 事务封装
- src/db/repositories.py: Match/Team/League/Prediction Repository

重构:
- 删除 src/data/match_lookup.py(由 Repository 替代)
- 数据源(bzzoiro/understat/injuries)不再自行 commit
- API 路由(ingest)改用 UnitOfWork
- LLM 服务(predict/orchestrator/eval/backtest)改用 UnitOfWork

事务边界统一由调用方控制,数据层不再自行决定 commit。

src/llm/predict.py

@@ -13,6 +13,7 @@ from threading import Lock
 from src.core.config import settings
 from src.db.base import AsyncSessionLocal
 from src.db.models import Match, Prediction
+from src.db.unit_of_work import get_uow
 from src.llm.context_builder import build_context
 from src.llm.provider import LLMProvider, get_default_provider
 
@@ -143,8 +144,9 @@ async def _predict_single(
     except Exception as e:
         raise RuntimeError(f"LLM 输出校验失败: {e}")
 
-    # 4. 存预测(独立 session,因为 context 用的是自己的 session)
-    async with AsyncSessionLocal() as db:
+    # 4. 存预测(使用 UnitOfWork 统一事务)
+    async with get_uow() as uow:
+        db = uow.session
         # 验证 match 存在
         m = await db.get(Match, match_id)
         if m is None:
@@ -188,4 +190,5 @@ async def _predict_single(
 
         # 5. 写入缓存
         _set_cached(match_id, settings.LLM_PROVIDER, provider.model, version, result)
+        await uow.commit()
         return result