fix: handle None created_at in lead_flow_postgres to prevent isoformat AttributeError

backend/common/lead_flow_postgres.py

@@ -169,7 +169,7 @@ async def get_lead_history(
                 "id": r["id"],
                 "is_human": r["is_human"],
                 "message": r["message"],
-                "timestamp": r["created_at"] if isinstance(r["created_at"], str) else r["created_at"].isoformat(),
+                "timestamp": (r["created_at"] if isinstance(r["created_at"], str) else r["created_at"].isoformat()) if r["created_at"] else "",
                 "conversation_id": conv_id,
             }
             for r in rows
@@ -199,7 +199,7 @@ async def get_lead_history(
                 "id": r["id"],
                 "is_human": r["is_human"],
                 "message": r["message"],
-                "timestamp": r["created_at"].isoformat(),
+                "timestamp": r["created_at"].isoformat() if r["created_at"] else "",
                 "conversation_id": conv_id,
             }
             for r in rows
@@ -238,8 +238,8 @@ async def list_lead_conversations(device_id: str, limit: int = 20) -> list[dict]
         return [
             {
                 "conv_id": r["conv_id"],
-                "started_at": r["started_at"] if isinstance(r["started_at"], str) else r["started_at"].isoformat(),
-                "last_at": r["last_at"] if isinstance(r["last_at"], str) else r["last_at"].isoformat(),
+                "started_at": (r["started_at"] if isinstance(r["started_at"], str) else r["started_at"].isoformat()) if r["started_at"] else "",
+                "last_at": (r["last_at"] if isinstance(r["last_at"], str) else r["last_at"].isoformat()) if r["last_at"] else "",
                 "turn_count": r["turn_count"],
             }
             for r in rows
@@ -271,8 +271,8 @@ async def list_lead_conversations(device_id: str, limit: int = 20) -> list[dict]
         return [
             {
                 "conv_id": r["conv_id"],
-                "started_at": r["started_at"].isoformat(),
-                "last_at": r["last_at"].isoformat(),
+                "started_at": r["started_at"].isoformat() if r["started_at"] else "",
+                "last_at": r["last_at"].isoformat() if r["last_at"] else "",
                 "turn_count": r["turn_count"],
             }
             for r in rows
@@ -312,7 +312,7 @@ async def get_all_history_for_dashboard(limit: int = 200) -> list[dict]:
                 "ai_response": r["ai_response"],
                 "products": r["products"],
                 "lead_stage": r["lead_stage"],
-                "created_at": r["created_at"] if isinstance(r["created_at"], str) else r["created_at"].isoformat(),
+                "created_at": (r["created_at"] if isinstance(r["created_at"], str) else r["created_at"].isoformat()) if r["created_at"] else "",
             }
             for r in rows
         ]
@@ -346,7 +346,7 @@ async def get_all_history_for_dashboard(limit: int = 200) -> list[dict]:
                 "ai_response": r["ai_response"],
                 "products": r["products"],
                 "lead_stage": r["lead_stage"],
-                "created_at": r["created_at"].isoformat(),
+                "created_at": r["created_at"].isoformat() if r["created_at"] else "",
             }
             for r in rows
         ]