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

frontend/src/pages/Matches.tsx

@@ -26,7 +26,7 @@ interface Prediction {
   pred_home_goals: number | null
   pred_away_goals: number | null
   pred_1x2: string | null
-  confidence: number | null
+  subjective_confidence: number | null
   reasoning: string | null
   agent_outputs: AgentReport[] | null
   agent_weights: Record<string, number> | null
@@ -40,7 +40,7 @@ interface AgentReport {
   data_sufficiency: string
   analysis: string
   home_edge: number | null
-  confidence: number | null
+  subjective_confidence: number | null
   key_evidence: string[]
   exp_home_goals: number | null
   exp_away_goals: number | null
@@ -239,7 +239,7 @@ export default function Matches() {
             <div className="bg-green-50 rounded p-3">
               <div className="text-gray-500 text-xs">置信度</div>
               <div className="text-xl font-bold">
-                {prediction.confidence !== null ? `${(prediction.confidence * 100).toFixed(0)}%` : '-'}
+                {prediction.subjective_confidence !== null ? `${(prediction.subjective_confidence * 100).toFixed(0)}%` : '-'}
               </div>
             </div>
           </div>
@@ -271,7 +271,7 @@ export default function Matches() {
                           主队优势 {r.home_edge > 0 ? '+' : ''}{r.home_edge.toFixed(2)}
                         </span>
                       )}
-                      {r.confidence !== null && <span>信心 {(r.confidence * 100).toFixed(0)}%</span>}
+                      {r.subjective_confidence !== null && <span>信心 {(r.subjective_confidence * 100).toFixed(0)}%</span>}
                       {r.probable_score && <span>比分 {r.probable_score}</span>}
                     </span>
                   </summary>