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/download/cache.rs

@@ -389,14 +389,18 @@ mod tests {
 
     #[test]
     fn test_queue_precache_config() {
-        let mut config = CacheConfig::default();
-        config.queue_precache_enabled = false;
+        let config = CacheConfig {
+            queue_precache_enabled: false,
+            ..CacheConfig::default()
+        };
 
         let cache = SmartCache::new(config);
         assert!(!cache.should_precache_queue());
 
-        let mut new_config = CacheConfig::default();
-        new_config.wifi_only = false;
+        let new_config = CacheConfig {
+            wifi_only: false,
+            ..CacheConfig::default()
+        };
         cache.update_config(new_config);
 
         assert!(cache.should_precache_queue());
@@ -407,9 +411,11 @@ mod tests {
         // wifi_only must not short-circuit precaching: the network gate lives in
         // the download pump, which checks the *actual* transport. Enabling
         // WiFi-only while on WiFi should still precache.
-        let mut config = CacheConfig::default();
-        config.queue_precache_enabled = true;
-        config.wifi_only = true;
+        let config = CacheConfig {
+            queue_precache_enabled: true,
+            wifi_only: true,
+            ..CacheConfig::default()
+        };
 
         let cache = SmartCache::new(config);
         assert!(cache.should_precache_queue());
@@ -422,7 +428,7 @@ mod tests {
     /// TRACES: UR-071 | DR-127 | UT-120
     #[tokio::test]
     async fn test_reclaim_expired_only_takes_expired_temporary_entries() {
-        use crate::storage::db_service::{DatabaseService, RusqliteService};
+        use crate::storage::db_service::RusqliteService;
         use rusqlite::Connection;
         use std::sync::{Arc, Mutex};