Fix critical data loss bugs and improve error handling

## Critical Fixes
- Fix CRITICAL-1: Empty scans no longer delete entire library
  - Use timestamp-based tombstone logic instead of path-based
  - Only mark missing if livePaths is not empty

- Fix CRITICAL-2: Re-scanned files now return from deleted state
  - Add deleted=false to SongsCompanion.insert in both scanners

## High Priority Fixes
- HIGH-1: Fix SQLite variable limit crash on large libraries
  - Replace isNotIn() with timestamp comparison (O(1) not O(n))

- HIGH-2: Fix UI freezing during scan
  - Throttle progress updates (every 50 files or end of scan)

- HIGH-3: Add error handling for scan failures
  - Wrap scanFolders/scanAndroidMediaStore in try/catch
  - Display scanError banner in LibraryScreen

- HIGH-4: Add error handling for playback failures
  - Wrap loadPlaylist in try/catch

## Medium Priority Fixes
- MEDIUM-1: Replace ! with ?? to handle unknown ProcessingState
- MEDIUM-2: Escape LIKE wildcards in search queries
- MEDIUM-4: Display StreamBuilder errors instead of treating as empty
- MEDIUM-8: Add tooltips to all IconButtons for accessibility
- Add doc comments to public database APIs

## Testing
- Add regression tests for CRITICAL-1 and CRITICAL-2
- All 9 existing tests pass

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/player/audio_handler.dart

@@ -34,8 +34,12 @@ class MeloAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandler {
     final sources = items
         .map((item) => AudioSource.uri(Uri.parse(item.id), tag: item))
         .toList();
-    await _player.setAudioSources(sources, initialIndex: startIndex);
-    play();
+    try {
+      await _player.setAudioSources(sources, initialIndex: startIndex);
+      await play();
+    } catch (e) {
+      playbackState.addError(e);
+    }
   }
 
   @override
@@ -97,7 +101,8 @@ class MeloAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandler {
         ProcessingState.buffering: AudioProcessingState.buffering,
         ProcessingState.ready: AudioProcessingState.ready,
         ProcessingState.completed: AudioProcessingState.completed,
-      }[_player.processingState]!,
+      }[_player.processingState] ??
+          AudioProcessingState.idle,
       playing: _player.playing,
       updatePosition: _player.position,
       bufferedPosition: _player.bufferedPosition,