Add an audible alert for incoming huddle requests

## Summary

Play a ringtone when another user invites someone to a huddle so incoming requests are noticeable even when Buzz is not in the foreground.

## Proposed behavior

- Enable the audible incoming-huddle alert for every Buzz user by default.
- Keep ringing until the recipient accepts, declines, or the request expires.
- Do not play the sound for the person who starts the huddle.
- Respect the app's mute state and the operating system's Do Not Disturb / notification controls.
- Add a per-user **Settings → Notifications → Huddle requests** toggle so users can disable the sound.
- Apply the behavior product-wide; this is not a community-admin setting.

## Acceptance criteria

- An eligible incoming huddle request starts the ringtone once and does not create overlapping playback loops for duplicate events.
- Accepting, declining, expiring, or cancelling the request stops playback immediately.
- Starting a huddle never rings the initiator's own client.
- Muted users and users who disable the huddle-request sound do not hear it.
- Existing huddle request UI and join/decline behavior remain unchanged.
- Automated tests cover start, stop, deduplication, initiator exclusion, and the disabled preference.

## Product context

Buzz already models huddle lifecycle as events, and this change adds client-side alert behavior for an incoming request. The feature should remain an explicit exception to Buzz's zero-notification default because a huddle request is synchronous and time-sensitive.

Start from Buzz's existing huddle lifecycle events, incoming request UI, and notification settings. Trace how join, decline, expiry, cancellation, mute state, and duplicate events are handled; done means the ringtone follows every acceptance criterion and automated tests cover start, stop, deduplication, initiator exclusion, and the disabled preference.

Written by the indexing model from the issue text.