refactoring: polished some files, .env.example update

repositories/transactions.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -17,7 +17,7 @@ class BalanceTXRepository:
         balance_after: int,
         description: str,
     ) -> BalanceTransaction:
-        created_at = datetime.now(timezone.UTC)
+        created_at = datetime.now(UTC)
         tx = BalanceTransaction(
             user_id=user_id,
             amount=amount,