feat: Sprint 2 - 概率语义 + 轻量快照 + 数据库约束

P0-05: confidence → subjective_confidence 改名(15 文件)
       LLM 主观置信度与概率分离
P0-02: predictions 增加 cutoff_at + input_hash(轻量快照)
       MatchContext 暴露 match_dt
       单/多 Agent 路径均记录快照元数据
P2-05: 数据库 CHECK 约束
       - pred_home_goals >= 0
       - pred_away_goals >= 0
       - subjective_confidence 0~1
       - pred_1x2 IN (1,X,2)
       - mode IN (single,multi)
P2-06: limit 分页约束(ge=1, le=200)
P2-01: 新增 /health/ready 就绪检查

src/api/routes/backtest.py

@@ -46,7 +46,7 @@ async def backtest(req: BacktestRequest):
             "scored": summary.scored,
             "accuracy_1x2": summary.accuracy_1x2,
             "avg_score_rmse": summary.avg_score_rmse,
-            "avg_confidence": summary.avg_confidence,
+            "avg_subjective_confidence": summary.avg_confidence,
             "calibration": summary.calibration,
         },
         "results": [
@@ -61,7 +61,7 @@ async def backtest(req: BacktestRequest):
                 "pred_home": r.pred_home,
                 "pred_away": r.pred_away,
                 "pred_1x2": r.pred_1x2,
-                "confidence": r.confidence,
+                "subjective_confidence": r.subjective_confidence,
                 "correct_1x2": r.correct_1x2,
             }
             for r in summary.results