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

tests/test_core.py

@@ -96,7 +96,7 @@ class TestProviderMock:
                 def raise_for_status(self): pass
                 def json(self):
                     return {
-                        "choices": [{"message": {"content": '{"pred_home_goals": 1.5, "pred_away_goals": 1.0, "1x2": "1", "confidence": 0.7, "reasoning": "test"}'}}],
+                        "choices": [{"message": {"content": '{"pred_home_goals": 1.5, "pred_away_goals": 1.0, "1x2": "1", "subjective_confidence": 0.7, "reasoning": "test"}'}}],
                         "usage": {"prompt_tokens": 100, "completion_tokens": 50},
                     }
             return FakeResp()