Register Telegram bot commands across default, private, and group scopes so
the slash-command menu is available outside DMs.
Changes from review feedback:
- Add asyncio.Lock to prevent race condition in _ensure_forum_commands
- Extract MAX_COMMANDS_PER_SCOPE constant (30) to avoid magic number
- Upgrade error logging from debug->warning in forum registration
- Add tests covering lazy forum registration and concurrent safety
- Remove /start handler from this PR (separate feature)
Fixes review: needs_work (race, magic number, log levels, missing tests)