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/library/library_service.dart

@@ -25,6 +25,7 @@ class LibraryService extends ChangeNotifier {
   int scanDone = 0;
   int scanTotal = 0;
   bool permissionDenied = false;
+  String? scanError;
 
   /// Auf Android: automatischer Geräte-Scan. Auf Desktop: Ordner wählen + scannen.
   Future<void> pickFolderAndScan() async {
@@ -61,6 +62,7 @@ class LibraryService extends ChangeNotifier {
     scanning = true;
     scanDone = 0;
     scanTotal = 0;
+    scanError = null;
     notifyListeners();
     try {
       if (Platform.isAndroid) {
@@ -69,6 +71,8 @@ class LibraryService extends ChangeNotifier {
         final folders = (await db.activeFolders()).map((f) => f.path).toList();
         await scanFolders(db, folders, onProgress: _onProgress);
       }
+    } catch (e) {
+      scanError = 'Scan fehlgeschlagen: $e';
     } finally {
       scanning = false;
       notifyListeners();