fix(github): use proper ISO timestamp instead of literal 'now()' string

_update_last_used was passing string 'now()' which PostgreSQL stores
literally instead of evaluating. Now uses datetime.now(timezone.utc).isoformat()
for proper timestamp storage.

Author: DevanshuNEU

backend/routes/github.py

@@ -117,10 +117,11 @@ async def _delete_github_connection(user_id: str) -> bool:
 async def _update_last_used(user_id: str) -> None:
     """Update last_used_at timestamp"""
     try:
+        from datetime import datetime, timezone
         from services.supabase_service import get_supabase_service
         db = get_supabase_service().client
         db.table("github_connections").update(
-            {"last_used_at": "now()"}
+            {"last_used_at": datetime.now(timezone.utc).isoformat()}
         ).eq("user_id", user_id).execute()
     except Exception:
         pass