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/storage/mod.rs

@@ -7,6 +7,7 @@ pub mod db_service;
 pub mod models;
 pub mod schema;
 
+use crate::utils::lock::MutexSafe;
 use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 
@@ -77,7 +78,7 @@ impl Database {
     /// Run all pending migrations
     pub fn migrate(&self) -> SqliteResult<()> {
         info!("Starting database migrations...");
-        let conn = self.conn.lock().unwrap();
+        let conn = self.conn.lock_safe();
 
         // Create migrations table if it doesn't exist
         debug!("Creating _migrations table if it doesn't exist...");
@@ -172,7 +173,7 @@ mod tests {
     fn test_migrations_run() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Check that tables exist
         let mut stmt = conn
@@ -186,7 +187,7 @@ mod tests {
     fn test_all_tables_created() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         let expected_tables = [
             "servers",
@@ -218,7 +219,7 @@ mod tests {
     fn test_fts_table_created() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         let exists: Option<String> = conn
             .query_row(
@@ -234,7 +235,7 @@ mod tests {
     fn test_server_crud() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Insert a server
         conn.execute(
@@ -282,7 +283,7 @@ mod tests {
     fn test_user_crud() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Create a server first (foreign key)
         conn.execute(
@@ -329,7 +330,7 @@ mod tests {
     fn test_cascade_delete_server_removes_users() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Create server and user
         conn.execute(
@@ -367,7 +368,7 @@ mod tests {
     fn test_item_insert_and_fts_search() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Create server first
         conn.execute(
@@ -424,7 +425,7 @@ mod tests {
     fn test_user_data_playback_position() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Setup: server, user, item
         conn.execute(
@@ -487,7 +488,7 @@ mod tests {
     fn test_sync_queue_operations() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Setup
         conn.execute(
@@ -552,7 +553,7 @@ mod tests {
     fn test_downloads_table() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Setup
         conn.execute(
@@ -627,7 +628,7 @@ mod tests {
 
         // Tables should still exist
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         let count: i32 = conn
             .query_row(
@@ -643,7 +644,7 @@ mod tests {
     fn test_global_active_user_deactivation() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Create two servers
         conn.execute(
@@ -713,7 +714,7 @@ mod tests {
     fn test_active_session_query_ordering() {
         let db = Database::open_in_memory().unwrap();
         let conn = db.connection();
-        let conn = conn.lock().unwrap();
+        let conn = conn.lock_safe();
 
         // Create server
         conn.execute(