chore(rust): clear the clippy backlog and finish the poison-tolerant lock sweep

`cargo clippy --all-targets` went from 51 warnings (23 in the lib) to zero.
Most were mechanical — needless borrows, `assert_eq!` against a bool literal,
`vec!` where an array does, `or_insert_with(Vec::new)`, a loop index used only
to index — and were applied with `clippy --fix`, then reviewed line by line.
That review caught one auto-fix that was *not* semantically neutral: dropping
the redundant `use hostname;` left its `#[cfg(target_os = "linux")]` orphaned
directly above `SERVICE_NAME`, which would have silently cfg'd the constant out
of every non-Linux build. Removed the stray attribute with the import.

Where a lint asked for a risky change rather than a better one, it is suppressed
with a comment saying why:

- `too_many_arguments` on five `#[tauri::command]` handlers and
  `ThumbnailCache::save_thumbnail` — most of the arity is `State<'_, _>`
  injection, and a parameter struct would change the IPC contract and the
  generated TypeScript for no readability gain.
- `large_enum_variant` on `PlayerStatusEvent` and `AutoplayDecision` — both are
  serde + specta wire types emitted a handful of times a second, never bulk
  allocated; boxing would have to stay invisible to the generated bindings while
  every match arm gained a deref.
- `await_holding_lock` on the `hybrid`/`offline` test modules — the guard is a
  test-only serialisation lock for the process-global `INCLUDE_CATALOG_BROWSE`
  flag, and the await it spans *is* the critical section. Each `#[tokio::test]`
  gets its own single-threaded runtime, so this is not the production deadlock
  class the lint targets; restructuring would reintroduce the flag race.

Real fixes elsewhere: `JellyfinItem::to_media_item` takes `self` by value, so it
is now `into_media_item`; the five-tuple episode row in the download commands
has a named `EpisodeRow` alias; the mpv `PropertyChange` arm matches
`name: "pause"` instead of guarding on it.

Also converted the last 27 raw `.lock().unwrap()` call sites to `lock_safe()`,
completing the `MutexSafe`/`RwLockSafe` convention. All of them turned out to be
in test modules — production code was already clean — so this is consistency
rather than a fix. The two raw locks in `utils/lock.rs` stay raw on purpose:
those tests deliberately poison a mutex to prove the helpers recover from it.

Pure refactoring: all 698 tests still pass.

src-tauri/src/player/mpv_backend_test.rs

@@ -1,14 +1,15 @@
-/// Tests for MpvBackend to prevent regressions
-///
-/// These tests are designed to catch common issues like:
-/// - Tokio runtime panics when spawning async tasks from std::thread
-/// - Position update thread failures
-/// - Event emission issues
-///
-/// TRACES: UR-003, UR-004 | IR-003 | IT-003, IT-004
+//! Tests for MpvBackend to prevent regressions
+//!
+//! These tests are designed to catch common issues like:
+//! - Tokio runtime panics when spawning async tasks from std::thread
+//! - Position update thread failures
+//! - Event emission issues
+//!
+//! TRACES: UR-003, UR-004 | IR-003 | IT-003, IT-004
 
 #[cfg(test)]
 mod tests {
+    use crate::utils::lock::MutexSafe;
     use std::sync::{Arc, Mutex};
     use tokio::sync::Mutex as TokioMutex;
 
@@ -67,14 +68,14 @@ mod tests {
             if let Ok(handle) = tokio::runtime::Handle::try_current() {
                 // Has runtime (shouldn't happen in this test)
                 handle.spawn(async move {
-                    *counter_clone.lock().unwrap() += 1;
+                    *counter_clone.lock_safe() += 1;
                 });
             } else {
                 // No runtime - use fallback (should happen in this test)
                 std::thread::spawn(move || {
                     let rt = tokio::runtime::Runtime::new().unwrap();
                     rt.block_on(async move {
-                        *counter_clone.lock().unwrap() += 1;
+                        *counter_clone.lock_safe() += 1;
                     });
                 });
             }
@@ -85,7 +86,7 @@ mod tests {
         // Wait for async task to complete
         std::thread::sleep(std::time::Duration::from_millis(100));
 
-        let count = *counter.lock().unwrap();
+        let count = *counter.lock_safe();
         assert_eq!(
             count, 1,
             "Fallback pattern should execute async code successfully"
@@ -109,13 +110,13 @@ mod tests {
                 let position = i as f64 * 0.25;
 
                 // Store position (simulating event emission)
-                positions_clone.lock().unwrap().push(position);
+                positions_clone.lock_safe().push(position);
             }
         });
 
         handle.join().unwrap();
 
-        let recorded_positions = positions.lock().unwrap();
+        let recorded_positions = positions.lock_safe();
         assert_eq!(
             recorded_positions.len(),
             5,