fix(decision-manager): AI 分析結果寫入 incidents.decision_chain (DB 長期保存)

修復 Gap: decision token 只有 Redis TTL 12min，AI 診斷歷史永久丟失
  - 新增 _persist_decision_to_db() method
  - get_or_create_decision() 完成後 fire-and-forget 寫入 PG
  - 寫入: ts / confidence / risk_level / provider / source / diagnosis[:200]
  - try/except 吞錯不影響主流程，warning log 追蹤

DB/Cache 分層:
  PG (長期): incidents.decision_chain (歷史) + outcomes + KM entries
  Redis (短期): decision token dedup + working memory + playbook cache

2026-04-16 Claude Sonnet 4.6 Asia/Taipei

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

apps/api/src/services/decision_manager.py

@@ -1271,6 +1271,14 @@ class DecisionManager:
         # 4. 儲存最終結果
         await self._save_token(token)
 
+        # 4b. 2026-04-16 Claude Sonnet 4.6: 將 AI 分析結果寫入 incidents.decision_chain (DB 長期保存)
+        # 修復 Gap: decision token 只有 Redis TTL ~12min，AI 分析結果歷史永久丟失
+        # 寫入: diagnosis / confidence / risk_level / provider / source
+        if token.proposal_data:
+            _fire_and_forget(
+                self._persist_decision_to_db(incident.incident_id, token.proposal_data)
+            )
+
         # 5. ADR-030 Phase 4: 自動執行判斷
         if token.state == DecisionState.READY and token.proposal_data:
             # 評估是否可以自動執行
@@ -2290,6 +2298,68 @@ class DecisionManager:
 
         return None
 
+    async def _persist_decision_to_db(
+        self, incident_id: str, proposal_data: dict
+    ) -> None:
+        """
+        2026-04-16 Claude Sonnet 4.6: 將 AI 分析結果寫入 incidents.decision_chain (PostgreSQL)
+        修復 Gap: decision token 只有 Redis TTL，AI 分析歷史永久丟失
+
+        寫入格式: JSON array，每次分析追加一條記錄
+        欄位: ts / confidence / risk_level / provider / source / diagnosis (前 200 字)
+        """
+        try:
+            from src.db.base import get_db_context
+            from sqlalchemy import text as _sa_text
+            import json as _json
+            from src.utils.timezone import now_taipei
+
+            entry = {
+                "ts": now_taipei().isoformat(),
+                "confidence": proposal_data.get("confidence", 0.0),
+                "risk_level": proposal_data.get("risk_level", "unknown"),
+                "provider": proposal_data.get("provider", proposal_data.get("source", "")),
+                "source": proposal_data.get("source", ""),
+                "diagnosis": str(proposal_data.get("diagnosis", proposal_data.get("description", "")))[:200],
+            }
+
+            async with get_db_context() as db:
+                # 讀取現有 decision_chain (可能為 null 或 json array)
+                r = await db.execute(
+                    _sa_text(
+                        "SELECT decision_chain FROM incidents WHERE incident_id = :iid"
+                    ),
+                    {"iid": incident_id},
+                )
+                row = r.fetchone()
+                if row is None:
+                    return
+                existing = row[0] or []
+                if isinstance(existing, str):
+                    try:
+                        existing = _json.loads(existing)
+                    except Exception:
+                        existing = []
+                if not isinstance(existing, list):
+                    existing = []
+                existing.append(entry)
+
+                await db.execute(
+                    _sa_text(
+                        "UPDATE incidents SET decision_chain = :dc::json WHERE incident_id = :iid"
+                    ),
+                    {"dc": _json.dumps(existing), "iid": incident_id},
+                )
+                await db.commit()
+                logger.debug(
+                    "decision_chain_persisted",
+                    incident_id=incident_id,
+                    confidence=entry["confidence"],
+                    provider=entry["provider"],
+                )
+        except Exception as e:
+            logger.warning("decision_chain_persist_failed", incident_id=incident_id, error=str(e))
+
     async def _save_token(self, token: DecisionToken) -> None:
         """儲存決策令牌到 Redis"""
         import json