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/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() =>