feat(GAP-D5 Task 4.2): Postmortem 自動組裝 hook

incident_service.resolve_incident() 結尾 fire-and-forget 呼叫
report_generation_service.trigger_postmortem()，補完孤兒服務的觸發路徑。

觸發條件（由 trigger_postmortem 內部判斷）:
- duration > POSTMORTEM_MIN_DURATION_MINUTES (10min)
- 含 AI root_cause / resolution_action / provider / auto_repaired

背景:
- report_generation_service.py 539 行服務於先前 session 建立
- main.py:322 已啟動 run_daily_report_loop（Task 4.1 ✅）
- trigger_postmortem 在 src/ 下無呼叫方 → 本 commit 補上

MASTER 藍圖 Phase 4 至此完整收官：
✅ Task 4.1 日度巡檢報告（08:00 台北排程，生產環境已跑）
✅ Task 4.2 Postmortem 自動組裝（本 commit 接上 resolve hook）

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

apps/api/src/services/incident_service.py

@@ -1002,6 +1002,45 @@ class IncidentService:
         except Exception as _disp_e:
             logger.warning("disposition_manual_resolve_failed", error=str(_disp_e))
 
+        # MASTER Task 4.2 (2026-04-14 Claude Sonnet 4.6): Postmortem 自動組裝
+        # Incident duration > POSTMORTEM_MIN_DURATION_MINUTES(10min) 時自動生成
+        # 孤兒 report_generation_service.trigger_postmortem 本次接上 resolve 路徑
+        try:
+            import asyncio
+            from src.services.report_generation_service import get_report_generation_service
+
+            alertname = (
+                incident.signals[0].labels.get("alertname", "UnknownAlert")
+                if incident.signals else "UnknownAlert"
+            )
+            title = f"{alertname} — {', '.join(incident.affected_services or ['N/A'])}"
+            root_cause = None
+            resolution_action = None
+            ai_provider = None
+            auto_repaired = False
+            if incident.decision_chain:
+                root_cause = incident.decision_chain.hypothesis
+                ai_provider = incident.decision_chain.model_used
+            if incident.outcome:
+                resolution_action = (incident.outcome.learning_notes or None)
+                auto_repaired = bool(incident.outcome.execution_success)
+
+            asyncio.create_task(
+                get_report_generation_service().trigger_postmortem(
+                    incident_id=incident.incident_id,
+                    title=title,
+                    created_at=incident.signals[0].fired_at if incident.signals else incident.resolved_at,
+                    resolved_at=incident.resolved_at,
+                    root_cause=root_cause,
+                    resolution_action=resolution_action,
+                    ai_provider=ai_provider,
+                    auto_repaired=auto_repaired,
+                )
+            )
+        except Exception as _pm_e:
+            logger.exception("postmortem_trigger_failed",
+                             incident_id=incident_id, error=str(_pm_e))
+
         return incident
 
     async def find_by_proposal_id(self, proposal_id: str) -> Incident | None: