fix: add created_by_username parameter to BountyService.add_bounty

The add_bounty method now accepts created_by_username parameter
to store the creator's username for display purposes.

core/services.py

@@ -124,6 +124,7 @@ class BountyService:
         text: Optional[str] = None,
         link: Optional[str] = None,
         due_date_ts: Optional[int] = None,
+        created_by_username: Optional[str] = None,
     ) -> Bounty:
         """Add a new bounty to the room. Requires admin permission."""
         if not self.is_admin(room_id, user_id):
@@ -141,6 +142,7 @@ class BountyService:
         bounty = Bounty(
             id=room_data.next_id,
             created_by_user_id=user_id,
+            created_by_username=created_by_username,
             text=text,
             link=link,
             due_date_ts=due_date_ts,