refactor: replace unwrap() with expect for mutex locks

Replace all `lock().unwrap()` calls on Mutex guards with explicit error messages using `.expect("... mutex poisoned")`. This improves robustness by providing clear diagnostics when a thread panics while holding the lock, preventing silent failures. Affected modules: events.rs (Renderer/MediaServer), all renderer backends, queue backend/implementation files. Also adds documentation to marker traits (HasQueue、 HasContinuousStream) and clarifies error handling in watcher loop with panic catching.

pmocontrol/src/queue/mod.rs

@@ -6,7 +6,7 @@ mod snapshot;
 
 use std::sync::{Arc, Mutex};
 
-pub use backend::{EnqueueMode, HasQueue, QueueBackend};
+pub use backend::{EnqueueMode, QueueBackend};
 pub use music_queue::{MusicQueue, SyncScheduleOutcome};
 pub use snapshot::{PlaybackItem, QueueSnapshot};