Fix incorrect timezone assignment in event_date validator

Updated the validation logic to correctly replace the timezone for `event_date`. This ensures proper handling of datetime objects without tzinfo.

backend/app/schemas/events.py

@@ -84,7 +84,7 @@ class EventResponse(EventInDBBase):
     @field_validator('event_date')
     def validate_datetime(cls, v):
         if v.tzinfo is None:
-           v.event_date =  v.event_date.replace(tzinfo=timezone.utc)
+           v =  v.replace(tzinfo=timezone.utc)
         return v