feat: add deleted_at, created_by_username to Bounty; timezone, admin_user_ids to RoomData

Issue #41: Model updates for Phase 2 features

Bounty model:
- Add deleted_at: int | None - timestamp when deleted (soft-delete)
- Add created_by_username: str | None - username for display purposes

RoomData model:
- Add timezone: str | None - room's timezone (e.g., "Asia/Jakarta")
- Add admin_user_ids: list[int] - list of admin user IDs

Storage adapter updated to handle new fields in load/save operations.
Tests added for new fields.

adapters/storage/json_file.py

@@ -56,6 +56,8 @@ class JsonFileRoomStorage:
                 due_date_ts=b.get("due_date_ts"),
                 created_at=b["created_at"],
                 created_by_user_id=b["created_by_user_id"],
+                deleted_at=b.get("deleted_at"),
+                created_by_username=b.get("created_by_username"),
             )
             for b in data.get("bounties", [])
         ]
@@ -64,6 +66,8 @@ class JsonFileRoomStorage:
             room_id=data["room_id"],
             bounties=bounties,
             next_id=data["next_id"],
+            timezone=data.get("timezone"),
+            admin_user_ids=data.get("admin_user_ids", []),
         )
 
     def save(self, room_data: RoomData) -> None:
@@ -71,6 +75,8 @@ class JsonFileRoomStorage:
         data = {
             "room_id": room_data.room_id,
             "next_id": room_data.next_id,
+            "timezone": room_data.timezone,
+            "admin_user_ids": room_data.admin_user_ids or [],
             "bounties": [
                 {
                     "id": b.id,
@@ -79,6 +85,8 @@ class JsonFileRoomStorage:
                     "due_date_ts": b.due_date_ts,
                     "created_at": b.created_at,
                     "created_by_user_id": b.created_by_user_id,
+                    "deleted_at": b.deleted_at,
+                    "created_by_username": b.created_by_username,
                 }
                 for b in room_data.bounties
             ],