feat: add category command handlers

Commands layer for category feature (Issue #87):

New Commands:
- /category - list all categories
- /category add <slug> <name> - create category (admin)
- /category delete <slug> - soft delete category (admin)

Updated Commands:
- /add - supports -cat <slug>[,<slug>] flag
- /update - supports -cat, -cat -, and -remove-cat flags
- /bounty - supports -c <slug>[,<slug>] filter
- /show - displays categories for bounty
- /help - includes category commands

Syntax Examples:
- /add Fix bug github.com/repo -cat bug,urgent
- /update 1 -cat feature,docs
- /update 1 -cat - (clear categories)
- /update 1 -remove-cat bug
- /bounty -c bug
- /bounty -c bug,feature

apps/telegram-bot/bot.py

@@ -15,6 +15,7 @@ from commands import (
     cmd_add,
     cmd_admin,
     cmd_bounty,
+    cmd_category,
     cmd_delete,
     cmd_delete_message,
     cmd_edit,
@@ -61,6 +62,7 @@ def build_app() -> Application:
     app.add_handler(CommandHandler("timezone", cmd_timezone))
     app.add_handler(CommandHandler("admin", cmd_admin))
     app.add_handler(CommandHandler("recover", cmd_recover))
+    app.add_handler(CommandHandler("category", cmd_category))
 
     app.add_handler(CallbackQueryHandler(cmd_delete_message))
 
@@ -82,6 +84,7 @@ async def post_init(app: Application) -> None:
             ("timezone", "Get/set room timezone"),
             ("admin", "Manage admins"),
             ("recover", "Recover deleted bounties"),
+            ("category", "Manage categories"),
             ("help", "Show help"),
         ]
     )