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/commands/repository.rs

@@ -3,6 +3,7 @@
 //!
 //! TRACES: UR-007, UR-035, UR-036 | JA-004, JA-005, JA-029, JA-030, JA-031
 
+use crate::utils::lock::MutexSafe;
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 
@@ -26,17 +27,17 @@ impl RepositoryManager {
     }
 
     pub fn create(&self, handle: String, repository: HybridRepository) {
-        let mut repos = self.repositories.lock().unwrap();
+        let mut repos = self.repositories.lock_safe();
         repos.insert(handle, Arc::new(repository));
     }
 
     pub fn get(&self, handle: &str) -> Option<Arc<HybridRepository>> {
-        let repos = self.repositories.lock().unwrap();
+        let repos = self.repositories.lock_safe();
         repos.get(handle).cloned()
     }
 
     pub fn destroy(&self, handle: &str) {
-        let mut repos = self.repositories.lock().unwrap();
+        let mut repos = self.repositories.lock_safe();
         repos.remove(handle);
     }
 }