feat: enhance expense service with timezone-aware datetime and improve balance retrieval

Author: Devasy

backend/app/expenses/service.py

@@ -1,5 +1,5 @@
 from collections import defaultdict
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Any, Dict, List, Optional
 
 from app.config import logger
@@ -1093,7 +1093,7 @@ async def get_friends_balance_summary(self, user_id: str) -> Dict[str, Any]:
 
         for result in results:
             friend_id = result["_id"]
-            total_balance = result["totalBalance"]
+            total_balance = result.get("totalBalance", 0)
 
             # Get friend details from map
             friend_details = friends_map.get(friend_id)
@@ -1129,7 +1129,9 @@ async def get_friends_balance_summary(self, user_id: str) -> Dict[str, Any]:
                 "netBalance": round(total_balance, 2),
                 "owesYou": total_balance > 0,
                 "breakdown": breakdown,
-                "lastActivity": datetime.utcnow(),  # TODO: Calculate actual last activity
+                "lastActivity": datetime.now(
+                    timezone.utc
+                ),  # TODO: Calculate actual last activity
             }
 
             friends_balance.append(friend_data)

backend/tests/expenses/test_expense_service.py

@@ -1,5 +1,6 @@
 import asyncio
 from datetime import datetime, timedelta, timezone
+from typing import Any
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -1612,7 +1613,9 @@ async def test_get_friends_balance_summary_success(expense_service):
     #   Group Beta: Main owes Friend2 70 (balance: -70 for Main)
     #   Total for Friend2: -70 (Main owes Friend2 70)
 
-    def sync_mock_settlements_aggregate_cursor_factory(pipeline, *args, **kwargs):
+    def sync_mock_settlements_aggregate_cursor_factory(
+        _pipeline: Any, *_args: Any, **_kwargs: Any
+    ) -> AsyncMock:
         # The optimized version returns aggregated results for all friends in one go
         mock_agg_cursor = AsyncMock()
         mock_agg_cursor.to_list.return_value = [