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/repository/hybrid.rs

@@ -5,6 +5,8 @@
 // @req: DR-012 - Local database for media metadata cache
 // @req: DR-013 - Repository pattern for online/offline data access
 
+#[cfg(test)]
+use crate::utils::lock::MutexSafe;
 use std::sync::Arc;
 
 use async_trait::async_trait;
@@ -546,16 +548,16 @@ mod tests {
         }
 
         fn get_query_count(&self) -> usize {
-            *self.query_count.lock().unwrap()
+            *self.query_count.lock_safe()
         }
 
         fn get_save_count(&self) -> usize {
-            *self.save_count.lock().unwrap()
+            *self.save_count.lock_safe()
         }
 
         async fn save_to_cache(&self, _parent_id: &str, items: &[MediaItem]) -> Result<usize, RepoError> {
-            *self.save_count.lock().unwrap() += 1;
-            *self.items.lock().unwrap() = items.to_vec();
+            *self.save_count.lock_safe() += 1;
+            *self.items.lock_safe() = items.to_vec();
             Ok(items.len())
         }
     }
@@ -567,8 +569,8 @@ mod tests {
         }
 
         async fn get_items(&self, _parent_id: &str, _options: Option<GetItemsOptions>) -> Result<SearchResult, RepoError> {
-            *self.query_count.lock().unwrap() += 1;
-            let items = self.items.lock().unwrap().clone();
+            *self.query_count.lock_safe() += 1;
+            let items = self.items.lock_safe().clone();
             let count = items.len();
             Ok(SearchResult {
                 items,
@@ -715,7 +717,7 @@ mod tests {
         }
 
         fn get_query_count(&self) -> usize {
-            *self.query_count.lock().unwrap()
+            *self.query_count.lock_safe()
         }
     }
 
@@ -726,7 +728,7 @@ mod tests {
         }
 
         async fn get_items(&self, _parent_id: &str, _options: Option<GetItemsOptions>) -> Result<SearchResult, RepoError> {
-            *self.query_count.lock().unwrap() += 1;
+            *self.query_count.lock_safe() += 1;
             Ok(SearchResult {
                 items: self.items.clone(),
                 total_record_count: self.items.len(),