Remove Lavaplayer and migrate to Lavalink

Replaced the bot's audio handling system by removing Lavaplayer dependencies and migrating to a Lavalink v4 node. Updated `MusicManager`, `TrackScheduler`, and related classes to utilize the `LavalinkClient`. Refactored playlist and track loading logic, task integrations, and event dispatch to integrate with the remote node. Deprecated the custom Udio source, preserving references for a future plugin port. Documented configuration and deployment details for the new audio architecture.

docs/threading.md

@@ -37,7 +37,8 @@ startup by [`TaskManager.init()`](tasks.md#taskmanager)):
 - `cancelAll()` — cancels every scheduled task; called first in
   [`App.shutdown()`](architecture.md#shutdown-lifecycle).
 
-The existing scheduled tasks are `MusicManagerCleanup`, `MusicPlayerAutoDisconnect`, and
+The existing scheduled tasks are `MusicManagerCleanup`, `MusicPlayerAutoDisconnect` (which reads the
+bot's gateway voice state rather than JDA's audio manager, see [music.md](music.md)), and
 `TaskCleanup` (see the [task catalog](tasks.md#concrete-task-catalog)).
 
 > **Direction of travel:** the scheduled-task abstraction is intended to eventually replace ad-hoc