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_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();