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/backend.rs

@@ -28,15 +28,11 @@
 //!   - This identity is used by the sync helpers to preserve the current
 //!     track across queue rebuilds when the MediaServer content changes.
 
+use crate::music_renderer::HasQueue;
 use crate::queue::MusicQueue;
 use crate::{errors::ControlPointError, PlaybackItem, QueueSnapshot};
 use std::sync::{atomic::AtomicBool, Arc, Mutex};
 
-/// Trait for types that have aMusicQueue.
-pub trait HasQueue {
-    fn queue(&self) -> &Arc<Mutex<MusicQueue>>;
-}
-
 /// Blanket implementation of QueueBackend for types that have a queue.
 /// All methods simply delegate to the underlying MusicQueue.
 impl<T: HasQueue> QueueBackend for T {