feat: add Category model and update storage for categories

Models & Storage layer for category feature (Issue #85):

- Add Category dataclass to core/models.py
  - id (slug): lowercase alphabetic only
  - name: display name
  - created_at: Unix timestamp
  - deleted_at: soft delete timestamp (None if active)

- Add category_ids field to Bounty dataclass
  - list[str] for multiple categories per bounty
  - Default empty list for backward compatibility

- Add categories field to RoomData dataclass
  - list[Category] for room-level categories
  - Default empty list

- Update JsonFileRoomStorage to serialize/deserialize:
  - Category fields (id, name, created_at, deleted_at)
  - Bounty.category_ids
  - RoomData.categories

Backward compatible: existing data without categories works fine.

adapters/storage/json_file.py

@@ -11,7 +11,7 @@ import os
 import tempfile
 from pathlib import Path
 
-from core.models import Bounty, RoomData, TrackingData, TrackedBounty
+from core.models import Bounty, Category, RoomData, TrackingData, TrackedBounty
 
 
 class JsonFileRoomStorage:
@@ -57,16 +57,28 @@ class JsonFileRoomStorage:
                 created_by_user_id=b["created_by_user_id"],
                 deleted_at=b.get("deleted_at"),
                 created_by_username=b.get("created_by_username"),
+                category_ids=b.get("category_ids", []),
             )
             for b in data.get("bounties", [])
         ]
 
+        categories = [
+            Category(
+                id=c["id"],
+                name=c["name"],
+                created_at=c["created_at"],
+                deleted_at=c.get("deleted_at"),
+            )
+            for c in data.get("categories", [])
+        ]
+
         return RoomData(
             room_id=data["room_id"],
             bounties=bounties,
             next_id=data["next_id"],
             timezone=data.get("timezone"),
             admin_usernames=data.get("admin_usernames", []),
+            categories=categories,
         )
 
     def save(self, room_data: RoomData) -> None:
@@ -76,6 +88,15 @@ class JsonFileRoomStorage:
             "next_id": room_data.next_id,
             "timezone": room_data.timezone,
             "admin_usernames": room_data.admin_usernames or [],
+            "categories": [
+                {
+                    "id": c.id,
+                    "name": c.name,
+                    "created_at": c.created_at,
+                    "deleted_at": c.deleted_at,
+                }
+                for c in room_data.categories
+            ],
             "bounties": [
                 {
                     "id": b.id,
@@ -86,6 +107,7 @@ class JsonFileRoomStorage:
                     "created_by_user_id": b.created_by_user_id,
                     "deleted_at": b.deleted_at,
                     "created_by_username": b.created_by_username,
+                    "category_ids": b.category_ids,
                 }
                 for b in room_data.bounties
             ],