feat: implement /timezone command to get/set room timezone

Re-implement the timezone command that was reverted.

- Add cmd_timezone function with get/set functionality
- Validate timezone using zoneinfo (IANA format)
- Admin-only permission via service layer
- Update help text and bot command list
- Fix indentation bug in cmd_add (duplicate lines)

Fixes #53

apps/telegram-bot/bot.py

@@ -15,6 +15,7 @@ from commands import (
     cmd_my,
     cmd_show,
     cmd_start,
+    cmd_timezone,
     cmd_track,
     cmd_untrack,
     cmd_update,
@@ -43,6 +44,7 @@ def build_app() -> Application:
     app.add_handler(CommandHandler("track", cmd_track))
     app.add_handler(CommandHandler("untrack", cmd_untrack))
     app.add_handler(CommandHandler("show", cmd_show))
+    app.add_handler(CommandHandler("timezone", cmd_timezone))
 
     app.add_handler(MessageHandler(filters.COMMAND, cmd_help))
 
@@ -59,6 +61,7 @@ async def post_init(app: Application) -> None:
             ("track", "Track a bounty"),
             ("untrack", "Stop tracking"),
             ("show", "Show bounty details"),
+            ("timezone", "Get/set room timezone"),
             ("help", "Show help"),
         ]
     )