Architecture remediation A/B/F: poison-tolerant locks, graceful backend init, doc fixes

Workstream A — poison-tolerant locking:
- Add utils/lock.rs with MutexSafe/RwLockSafe extension traits that recover a
  poisoned std::sync lock instead of panicking, plus unit tests.
- Replace all 153 .lock().unwrap() and 4 .read()/.write().unwrap() production
  sites with _safe variants across 14 files, eliminating the player
  crash-cascade class. Tokio async mutexes are unchanged.

Workstream B — graceful backend init:
- create_player_backend no longer panics when MPV/ExoPlayer fail to initialize;
  it falls back to NullBackend and emits a backend-init-failed event so the UI
  can show "playback unavailable" instead of the app crashing. Fatal DB-setup
  panics are kept.

Workstream F — doc reconciliation:
- Rewrite software-architecture.md's inaccurate "thin UI / ~800 lines" claims to
  reflect reality (~20.5k non-test frontend) and document the events+polling
  hybrid plus the new locking/backend-init behavior.

src-tauri/src/player/mpv_backend.rs

@@ -1,3 +1,4 @@
+use crate::utils::lock::MutexSafe;
 use log::{debug, error, info, warn};
 use super::backend::{PlayerBackend, PlayerError};
 use super::events::{PlayerEventEmitter, PlayerStatusEvent};
@@ -190,7 +191,7 @@ impl MpvBackend {
                         libmpv::events::Event::PlaybackRestart => {
                             debug!("[MpvBackend] Playback started/resumed");
 
-                            let media_id = state.lock().unwrap().current_media.as_ref().map(|m| m.id.clone());
+                            let media_id = state.lock_safe().current_media.as_ref().map(|m| m.id.clone());
 
                             if let Some(emitter) = &event_emitter {
                                 emitter.emit(PlayerStatusEvent::StateChanged {
@@ -202,7 +203,7 @@ impl MpvBackend {
                         libmpv::events::Event::PropertyChange { name, .. } if name == "pause" => {
                             // Handle pause state changes
                             if let Ok(is_paused) = mpv.get_property::<bool>("pause") {
-                                let media_id = state.lock().unwrap().current_media.as_ref().map(|m| m.id.clone());
+                                let media_id = state.lock_safe().current_media.as_ref().map(|m| m.id.clone());
 
                                 if let Some(emitter) = &event_emitter {
                                     emitter.emit(PlayerStatusEvent::StateChanged {
@@ -308,7 +309,7 @@ impl MpvBackend {
                     if !is_paused {
                         // Throttled progress reporting (every 30s)
                         let jellyfin_id = {
-                            let state = state_for_position.lock().unwrap();
+                            let state = state_for_position.lock_safe();
                             state.current_media.as_ref()
                                 .and_then(|m| m.jellyfin_id().map(|s| s.to_string()))
                         };
@@ -376,7 +377,7 @@ impl PlayerBackend for MpvBackend {
 
         // Update state
         {
-            let mut state = self.state.lock().unwrap();
+            let mut state = self.state.lock_safe();
             state.current_media = Some(media.clone());
         }
 
@@ -422,7 +423,7 @@ impl PlayerBackend for MpvBackend {
             message: format!("Failed to stop: {:?}", e),
         })?;
 
-        let mut state = self.state.lock().unwrap();
+        let mut state = self.state.lock_safe();
         state.current_media = None;
 
         Ok(())
@@ -460,7 +461,7 @@ impl PlayerBackend for MpvBackend {
                 message: format!("Failed to set volume: {:?}", e),
             })?;
 
-        let mut state = self.state.lock().unwrap();
+        let mut state = self.state.lock_safe();
         state.volume = clamped;
 
         Ok(())
@@ -480,7 +481,7 @@ impl PlayerBackend for MpvBackend {
     }
 
     fn state(&self) -> PlayerState {
-        let state = self.state.lock().unwrap();
+        let state = self.state.lock_safe();
 
         if let Some(ref media) = state.current_media {
             let is_paused = self.mpv.get_property::<bool>("pause").unwrap_or(true);
@@ -506,7 +507,7 @@ impl PlayerBackend for MpvBackend {
     }
 
     fn volume(&self) -> f32 {
-        let state = self.state.lock().unwrap();
+        let state = self.state.lock_safe();
         state.volume
     }