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/playback_reporting/throttle.rs

@@ -5,6 +5,7 @@
 
 #![allow(dead_code)]
 
+use crate::utils::lock::MutexSafe;
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, Instant};
@@ -34,7 +35,7 @@ impl EventThrottler {
 
     /// Checks if enough time has elapsed since the last report for this item
     pub fn should_report(&self, item_id: &str) -> bool {
-        let last_times = self.last_report_time.lock().unwrap();
+        let last_times = self.last_report_time.lock_safe();
 
         if let Some(last_time) = last_times.get(item_id) {
             let elapsed = last_time.elapsed();
@@ -54,7 +55,7 @@ impl EventThrottler {
 
     /// Marks the item as reported at the current time
     pub fn mark_reported(&self, item_id: &str) {
-        let mut last_times = self.last_report_time.lock().unwrap();
+        let mut last_times = self.last_report_time.lock_safe();
         last_times.insert(item_id.to_string(), Instant::now());
 
         log::debug!(
@@ -66,14 +67,14 @@ impl EventThrottler {
 
     /// Clears all tracked report times
     pub fn clear(&self) {
-        let mut last_times = self.last_report_time.lock().unwrap();
+        let mut last_times = self.last_report_time.lock_safe();
         last_times.clear();
         log::debug!("[EventThrottler] Cleared all tracked report times");
     }
 
     /// Removes a specific item from tracking
     pub fn clear_item(&self, item_id: &str) {
-        let mut last_times = self.last_report_time.lock().unwrap();
+        let mut last_times = self.last_report_time.lock_safe();
         last_times.remove(item_id);
         log::debug!("[EventThrottler] Cleared tracking for {}", item_id);
     }