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/android_scan.dart

@@ -44,11 +44,15 @@ Future<int> scanAndroidMediaStore(
 
     var coverPath = prev?.coverPath;
     if (coverPath == null) {
-      final art = await audioQuery.queryArtwork(s.id, ArtworkType.AUDIO, size: 512);
-      if (art != null && art.isNotEmpty) {
-        final f = File(p.join(covers.path, '$id.img'));
-        await f.writeAsBytes(art);
-        coverPath = f.path;
+      try {
+        final art = await audioQuery.queryArtwork(s.id, ArtworkType.AUDIO, size: 512);
+        if (art != null && art.isNotEmpty) {
+          final f = File(p.join(covers.path, '$id.img'));
+          await f.writeAsBytes(art);
+          coverPath = f.path;
+        }
+      } catch (_) {
+        // Kaputtes Cover verzögert nicht den Scan.
       }
     }
 
@@ -63,11 +67,17 @@ Future<int> scanAndroidMediaStore(
       dateAddedMs: prev?.dateAddedMs ??
           (s.dateAdded != null ? s.dateAdded! * 1000 : now),
       updatedAtMs: now,
+      deleted: const Value(false),
     ));
-    onProgress?.call(++done, songs.length);
+    ++done;
+    if (done % 50 == 0 || done == songs.length) {
+      onProgress?.call(done, songs.length);
+    }
   }
 
   await db.upsertSongs(companions);
-  await db.markMissing(livePaths, now);
+  if (livePaths.isNotEmpty) {
+    await db.markMissing(now);
+  }
   return companions.length;
 }

lib/library/database.dart

@@ -43,6 +43,7 @@ class MeloDb extends _$MeloDb {
   @override
   int get schemaVersion => 1;
 
+  /// Überwacht alle Songs (nicht getombstonte). Nach Titel sortiert.
   Stream<List<Song>> watchSongs() {
     return (select(songs)
           ..where((s) => s.deleted.equals(false))
@@ -50,6 +51,7 @@ class MeloDb extends _$MeloDb {
         .watch();
   }
 
+  /// Überwacht die zuletzt hinzugefügten Songs (nicht getombstonte).
   Stream<List<Song>> watchRecent({int limit = 50}) {
     return (select(songs)
           ..where((s) => s.deleted.equals(false))
@@ -60,33 +62,43 @@ class MeloDb extends _$MeloDb {
         .watch();
   }
 
+  /// Sucht Songs nach Titel, Künstler oder Album. Wildcards werden escaped.
   Stream<List<Song>> searchSongs(String query) {
-    final like = '%${query.toLowerCase()}%';
+    final escaped = query
+        .toLowerCase()
+        .replaceAll('\\', '\\\\')
+        .replaceAll('%', '\\%')
+        .replaceAll('_', '\\_');
+    final like = '%$escaped%';
     return (select(songs)
           ..where((s) =>
               s.deleted.equals(false) &
-              (s.title.lower().like(like) |
-                  s.artist.lower().like(like) |
-                  s.album.lower().like(like)))
+              (s.title.lower().like(like, escapeChar: '\\') |
+                  s.artist.lower().like(like, escapeChar: '\\') |
+                  s.album.lower().like(like, escapeChar: '\\')))
           ..orderBy([(s) => OrderingTerm(expression: s.title)]))
         .watch();
   }
 
+  /// Alle Songs inkl. getombstonte. Intern für ID-Stabilität beim Scan.
   Future<List<Song>> allSongs() => select(songs).get();
 
+  /// Upsert Songs nach Pfad. Neue Songs bekommen UUID; bekannte erhalten sie zurück.
+  /// Setzt [updatedAtMs] auf die Werte der Companions (normalerweise Scan-Startzeit).
   Future<void> upsertSongs(List<SongsCompanion> items) async {
     await batch((b) => b.insertAllOnConflictUpdate(songs, items));
   }
 
   /// Tombstone für Songs, deren Datei beim Scan nicht mehr gefunden wurde.
-  Future<void> markMissing(List<String> livePaths, int now) async {
-    final q = update(songs)..where((s) => s.deleted.equals(false));
-    if (livePaths.isNotEmpty) {
-      q.where((s) => s.path.isNotIn(livePaths));
-    }
-    await q.write(
-      SongsCompanion(deleted: const Value(true), updatedAtMs: Value(now)),
-    );
+  /// Löscht nur Songs, die in diesem Scan (Zeitstempel [now]) nicht aktualisiert wurden.
+  Future<void> markMissing(int now) async {
+    await (update(songs)
+          ..where((s) =>
+              s.deleted.equals(false) &
+              s.updatedAtMs.isSmallerThanValue(now)))
+        .write(
+          SongsCompanion(deleted: const Value(true), updatedAtMs: Value(now)),
+        );
   }
 
   Stream<List<Folder>> watchFolders() =>

lib/library/library_screen.dart

@@ -43,6 +43,17 @@ class LibraryScreen extends StatelessWidget {
                 ),
               ],
             ),
+          if (lib.scanError != null)
+            MaterialBanner(
+              backgroundColor: Colors.red.shade900,
+              content: Text(lib.scanError!),
+              actions: [
+                TextButton(
+                  onPressed: () => lib.scanError = null,
+                  child: const Text('Verwerfen'),
+                ),
+              ],
+            ),
           if (lib.scanning)
             Padding(
               padding: const EdgeInsets.all(12),
@@ -62,6 +73,12 @@ class LibraryScreen extends StatelessWidget {
             child: StreamBuilder<List<Song>>(
               stream: db.watchSongs(),
               builder: (context, snapshot) {
+                if (snapshot.hasError) {
+                  return Center(
+                    child: Text('Fehler: ${snapshot.error}',
+                        style: const TextStyle(color: Colors.white54)),
+                  );
+                }
                 final songs = snapshot.data ?? const [];
                 if (songs.isEmpty && !lib.scanning) {
                   return const _Empty();

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();

lib/library/scan_service.dart

@@ -68,9 +68,13 @@ Future<int> scanFolders(
     var coverPath = prev?.coverPath;
     final pics = meta?.pictures ?? const <Picture>[];
     if (pics.isNotEmpty && coverPath == null) {
-      final f = File(p.join(covers.path, '$id.img'));
-      await f.writeAsBytes(pics.first.bytes);
-      coverPath = f.path;
+      try {
+        final f = File(p.join(covers.path, '$id.img'));
+        await f.writeAsBytes(pics.first.bytes);
+        coverPath = f.path;
+      } catch (_) {
+        // Kaputtes Cover verzögert nicht den Scan.
+      }
     }
 
     companions.add(SongsCompanion.insert(
@@ -83,11 +87,17 @@ Future<int> scanFolders(
       coverPath: Value(coverPath),
       dateAddedMs: prev?.dateAddedMs ?? now,
       updatedAtMs: now,
+      deleted: const Value(false),
     ));
-    onProgress?.call(++done, files.length);
+    ++done;
+    if (done % 50 == 0 || done == files.length) {
+      onProgress?.call(done, files.length);
+    }
   }
 
   await db.upsertSongs(companions);
-  await db.markMissing(livePaths, now);
+  if (livePaths.isNotEmpty) {
+    await db.markMissing(now);
+  }
   return companions.length;
 }

lib/library/search_screen.dart

@@ -37,6 +37,12 @@ class _SearchScreenState extends State<SearchScreen> {
           : StreamBuilder<List<Song>>(
               stream: db.searchSongs(_query),
               builder: (context, snapshot) {
+                if (snapshot.hasError) {
+                  return Center(
+                    child: Text('Fehler: ${snapshot.error}',
+                        style: const TextStyle(color: Colors.white54)),
+                  );
+                }
                 final songs = snapshot.data ?? const [];
                 if (songs.isEmpty) {
                   return const Center(