fix(api): 簽核後更新 Incident 狀態為 RESOLVED

根因: 簽核成功後 Incident.status 未更新，導致刷新頁面後 Y/n 按鈕重現

修復:
- proposal_service.py: 新增 resolve_incident_after_approval() 方法
- approvals.py: sign_approval 成功後呼叫更新 Incident 狀態
- 使用 metadata.incident_id 反查關聯的 Incident

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

apps/api/src/services/proposal_service.py

@@ -501,6 +501,70 @@ class ProposalService:
                 error=str(e),
             )
 
+    # =========================================================================
+    # Phase 6.5: 簽核完成後更新 Incident 狀態
+    # =========================================================================
+
+    async def resolve_incident_after_approval(
+        self,
+        incident_id: str,
+        approval_id: str | None = None,
+    ) -> bool:
+        """
+        簽核完成後更新 Incident 狀態為 RESOLVED
+
+        當 Approval 達到所需簽核數時呼叫，更新:
+        1. incident.status → RESOLVED
+        2. incident.decision.state → completed (如果有)
+
+        Args:
+            incident_id: Incident ID
+            approval_id: 簽核的 Approval ID (用於日誌)
+
+        Returns:
+            是否更新成功
+        """
+        try:
+            incident = await self._load_incident(incident_id)
+            if not incident:
+                logger.warning(
+                    "resolve_incident_not_found",
+                    incident_id=incident_id,
+                    approval_id=approval_id,
+                )
+                return False
+
+            # 更新狀態
+            old_status = incident.status
+            incident.status = IncidentStatus.RESOLVED
+            incident.updated_at = datetime.now(timezone.utc)
+
+            # 更新 decision.state (如果有)
+            if incident.decision:
+                incident.decision.state = "completed"
+
+            # 持久化
+            await self._persist_incident(incident)
+
+            logger.info(
+                "incident_resolved_after_approval",
+                incident_id=incident_id,
+                approval_id=approval_id,
+                old_status=old_status.value,
+                new_status="resolved",
+            )
+
+            return True
+
+        except Exception as e:
+            logger.exception(
+                "resolve_incident_error",
+                incident_id=incident_id,
+                approval_id=approval_id,
+                error=str(e),
+            )
+            return False
+
 
 # =============================================================================
 # Singleton