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.

core/models.py

@@ -1,6 +1,20 @@
 """Domain dataclasses for JIGAIDO bounty tracker."""
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
+
+
+@dataclass
+class Category:
+    """A category for organizing bounties in a room.
+
+    Categories are per-room and support soft delete.
+    The id (slug) must be lowercase alphabetic only (e.g., "bug", "feature").
+    """
+
+    id: str  # slug: lowercase alphabetic only, e.g., "bug", "feature"
+    name: str  # display name: e.g., "Bug", "Feature"
+    created_at: int
+    deleted_at: int | None = None  # soft delete
 
 
 @dataclass
@@ -12,6 +26,8 @@ class Bounty:
 
     The deleted_at field indicates soft-delete: None means not deleted,
     a value means deleted at that Unix timestamp.
+
+    The category_ids field lists category slugs associated with this bounty.
     """
 
     id: int
@@ -22,6 +38,7 @@ class Bounty:
     created_by_user_id: int
     deleted_at: int | None = None
     created_by_username: str | None = None
+    category_ids: list[str] = field(default_factory=list)
 
 
 @dataclass
@@ -45,6 +62,7 @@ class RoomData:
 
     The timezone field stores the room's timezone (e.g., "Asia/Jakarta"), default UTC+0.
     The admin_usernames field lists usernames who have admin privileges in this room.
+    The categories field contains all categories for organizing bounties in this room.
     """
 
     room_id: int
@@ -52,10 +70,13 @@ class RoomData:
     next_id: int
     timezone: str | None = None
     admin_usernames: list[str] | None = None
+    categories: list[Category] = field(default_factory=list)
 
     def __post_init__(self):
         if self.admin_usernames is None:
             self.admin_usernames = []
+        if self.categories is None:
+            self.categories = []
 
 
 @dataclass