fix(api): incidents list 不再同步等待 AI 決策 (效能修復)

問題: GET /api/v1/incidents 對每個 incident await AI 分析 (120-180s)
      多個活躍 incident 時 timeout 乘積爆炸 → 前端完全無法載入

修復:
- list endpoint 只查 Redis 已快取的決策 token (立即返回)
- 無快取時回 decision=null，背景 fire-and-forget 觸發 AI
- 前端對有興趣的 incident 再 GET 單筆端點取得決策結果

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/api/v1/incidents.py

@@ -143,37 +143,46 @@ async def list_incidents() -> IncidentListResponse:
 
         incidents.sort(key=safe_created_at, reverse=True)
 
-        # Phase 6.5: 為每個事件生成決策令牌 (非同步並行)
+        # 2026-04-09 Claude Sonnet 4.6: 效能修復 — list endpoint 不同步等待 AI
+        # 原設計: 每個 incident await AI 決策 (120-180s timeout)，多 incident 時乘積爆炸
+        # 修復: 只取已存在的決策 token，若無則背景觸發生成，前端 poll 單筆 GET 取得結果
+        import asyncio
+
         responses = []
+        background_tasks = []
+
         for incident in incidents:
             try:
-                # P0/P1 給更短的 timeout (緊急)
-                # 2026-03-27 ogt: 增加超時 (Ollama CPU 模式 llama3.2:3b 約 2-3 分鐘)
-                timeout = 120.0 if incident.severity in (Severity.P0, Severity.P1) else 180.0
-
-                decision_token = await decision_manager.get_or_create_decision(
-                    incident=incident,
-                    timeout_sec=timeout,
-                )
-
-                decision_info = DecisionInfo(
-                    token=decision_token.token,
-                    state=decision_token.state.value,
-                    proposal_data=decision_token.proposal_data,
-                    proposal_id=decision_token.proposal_id,
-                )
-
-                responses.append(IncidentResponse.from_incident(incident, decision_info))
-
+                # 只查已快取的決策 (不等待 AI，立即返回)
+                existing = await decision_manager._find_existing_token(incident.incident_id)
+                if existing:
+                    decision_info = DecisionInfo(
+                        token=existing.token,
+                        state=existing.state.value,
+                        proposal_data=existing.proposal_data,
+                        proposal_id=existing.proposal_id,
+                    )
+                    responses.append(IncidentResponse.from_incident(incident, decision_info))
+                else:
+                    # 無快取 → 背景觸發，本次返回 None（前端看到 decision=null 會 poll）
+                    responses.append(IncidentResponse.from_incident(incident, None))
+                    timeout = 120.0 if incident.severity in (Severity.P0, Severity.P1) else 180.0
+                    background_tasks.append(
+                        decision_manager.get_or_create_decision(incident=incident, timeout_sec=timeout)
+                    )
             except Exception as e:
                 logger.warning(
                     "decision_generation_failed",
                     incident_id=incident.incident_id,
                     error=str(e),
                 )
-                # 即使決策生成失敗，也返回事件 (不含 decision)
                 responses.append(IncidentResponse.from_incident(incident, None))
 
+        # 背景觸發 AI 決策（fire-and-forget，不阻塞 response）
+        if background_tasks:
+            for task in background_tasks:
+                asyncio.create_task(task)
+
         logger.info(
             "incidents_listed",
             count=len(incidents),