fix: define tz variable in cmd_update before using it

The cmd_update function was missing the tz = ZoneInfo(timezone_str) conversion.
This caused "tz is not defined" error when confirming edits.

apps/telegram-bot/commands.py

@@ -418,6 +418,11 @@ async def cmd_update(update: Update, ctx: ContextTypes.DEFAULT_TYPE) -> None:
     room_id = get_room_id(update)
     timezone_str = BOUNTY_SERVICE.get_timezone(room_id)
 
+    try:
+        tz = ZoneInfo(timezone_str)
+    except (KeyError, ZoneInfoNotFoundError):
+        tz = ZoneInfo("UTC")
+
     text, link, due_date_ts, clear_link, clear_date = parse_args(args[1:], timezone_str)
     if (
         not text