Add an audible alert for incoming huddle requests

[Clyteroll]

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.