feat(api): Phase 6.5 Statistics API + Y/n 按鈕修復

新增:
- /stats/incidents/summary - 事件總覽統計
- /stats/incidents/resolution - 解決時間 P50/P95
- /stats/ai-performance - AI 提案效能
- /stats/services/affected - 受影響服務排名

修復:
- Y/n 按鈕永久禁用問題 (decision.state=completed 但 incident 未解決)
- decision_manager.py: 只有當 incident 也已解決才返回已完成的 decision

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/services/decision_manager.py

@@ -300,12 +300,23 @@ class DecisionManager:
 
         # 1. 檢查現有 token
         existing_token = await self._find_existing_token(incident.incident_id)
-        if existing_token and existing_token.state in (
-            DecisionState.READY,
-            DecisionState.EXECUTING,
-            DecisionState.COMPLETED,
-        ):
-            return existing_token
+        if existing_token:
+            # READY 或 EXECUTING 狀態: 直接返回
+            if existing_token.state in (DecisionState.READY, DecisionState.EXECUTING):
+                return existing_token
+            # COMPLETED 狀態: 只有 incident 也已解決才返回，否則創建新 decision
+            # 修復: 避免 incident 未解決但 decision 已完成導致 Y/n 按鈕永久禁用
+            if existing_token.state == DecisionState.COMPLETED:
+                from src.models.incident import IncidentStatus
+                if incident.status in (IncidentStatus.RESOLVED, IncidentStatus.CLOSED):
+                    return existing_token
+                # incident 仍在處理中，需要新的 decision
+                logger.info(
+                    "decision_reset_for_active_incident",
+                    token=existing_token.token,
+                    incident_id=incident.incident_id,
+                    incident_status=incident.status.value,
+                )
 
         # 2. 建立新 token
         token = DecisionToken(
@@ -539,12 +550,21 @@ class DecisionManager:
 
         # 檢查現有 token
         existing_token = await self._find_existing_token(incident.incident_id)
-        if existing_token and existing_token.state in (
-            DecisionState.READY,
-            DecisionState.EXECUTING,
-            DecisionState.COMPLETED,
-        ):
-            return existing_token
+        if existing_token:
+            # READY 或 EXECUTING 狀態: 直接返回
+            if existing_token.state in (DecisionState.READY, DecisionState.EXECUTING):
+                return existing_token
+            # COMPLETED 狀態: 只有 incident 也已解決才返回
+            if existing_token.state == DecisionState.COMPLETED:
+                from src.models.incident import IncidentStatus
+                if incident.status in (IncidentStatus.RESOLVED, IncidentStatus.CLOSED):
+                    return existing_token
+                logger.info(
+                    "decision_reset_for_active_incident_consensus",
+                    token=existing_token.token,
+                    incident_id=incident.incident_id,
+                    incident_status=incident.status.value,
+                )
 
         # 建立新 token
         token = DecisionToken(