fix(ai): 修復 _call_openclaw_analyze datetime 序列化失敗 → fallback Gemini

signals dict 內含 datetime 物件，httpx json= 無法序列化
加入 _to_serializable 遞迴轉換，datetime → str

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

apps/api/src/services/openclaw.py

@@ -276,12 +276,24 @@ class OpenClawService:
         """
         try:
             client = await self._get_client()
+            import json as _json
+            def _to_serializable(obj):
+                if isinstance(obj, dict):
+                    return {k: _to_serializable(v) for k, v in obj.items()}
+                if isinstance(obj, list):
+                    return [_to_serializable(i) for i in obj]
+                try:
+                    _json.dumps(obj)
+                    return obj
+                except (TypeError, ValueError):
+                    return str(obj)
+
             payload = {
                 "incident_id": incident_id,
                 "severity": severity,
-                "signals": signals[:5],
+                "signals": _to_serializable(signals[:5]),
                 "affected_services": affected_services,
-                "expert_context": expert_context,
+                "expert_context": _to_serializable(expert_context) if expert_context else None,
             }
             resp = await client.post(
                 f"{settings.OPENCLAW_URL}/api/v1/analyze/incident",