chore(flutter): drop success-path diagnostic prints

Cleanup pass on the noisy debugPrints we added during the recent
debugging sessions. Remaining prints are error-path only:
- AlbumCoverCache fetch failed
- album/playlist cold-cache fetch failed
- audio_handler playbackEventStream error
- audio_handler queue supersession (rare race)
- audio_handler forward/backward fill failed
- artist_detail play failed
- cacheFirst fetchAndPopulate failed (only when tag is set)

Removed per-event chatter:
- audio_handler player-state and processingState subscribers
- audio_handler timing measurements (built initial / setAudioSources
  / forward fill / backward fill)
- audio_handler cache hit/miss per-track (fired N times per play)
- audio_handler register stream cache (verifiable via Settings)
- audio_handler.configure log
- albumProvider step-by-step lines (drift miss / online / drift hit /
  album hit but no tracks)
- playlistDetailProvider step-by-step lines (calling get / drift
  write done / 404 evicted / drift hit / playlist hit but no tracks)
- playlistsListProvider wire returned + drift rows lines
- playTracks stage timings (serverUrl / token / configure / setQueue
  / play returned)
- metadataPrefetcher warming N artists
- artist_detail play tapped — N tracks success log
- cacheFirst step-by-step (drift hit / drift miss / online / done)

`tag` parameter on cacheFirst preserved for ad-hoc instrumentation.

flutter_client/lib/library/artist_detail_screen.dart

@@ -95,8 +95,6 @@ class ArtistDetailScreen extends ConsumerWidget {
                     try {
                       final tracks =
                           await ref.read(artistTracksProvider(id).future);
-                      debugPrint(
-                          'artist_detail: play tapped — ${tracks.length} tracks for $id');
                       if (tracks.isEmpty) {
                         if (context.mounted) {
                           ScaffoldMessenger.of(context).showSnackBar(

flutter_client/lib/library/library_providers.dart

@@ -187,11 +187,9 @@ final albumProvider = StreamProvider.family<
   Future<bool> fetchAndPopulate() async {
     try {
       final api = await ref.read(libraryApiProvider.future);
-      debugPrint('albumProvider($albumId): calling getAlbum');
       final fresh = await api
           .getAlbum(albumId)
           .timeout(const Duration(seconds: 10));
-      debugPrint('albumProvider($albumId): got ${fresh.tracks.length} tracks, writing to drift');
       // Collect every artist mentioned by the album + its tracks so
       // the JOINs that drive both the album header and the track rows
       // have something to bind to. Without this, drift returns null
@@ -224,7 +222,6 @@ final albumProvider = StreamProvider.family<
         b.insertAllOnConflictUpdate(db.cachedTracks,
             fresh.tracks.map((t) => t.toDrift()).toList());
       });
-      debugPrint('albumProvider($albumId): drift write done; awaiting watch re-emit');
       return true;
     } catch (e, st) {
       debugPrint('albumProvider($albumId): cold-cache fetch failed: $e\n$st');
@@ -235,7 +232,6 @@ final albumProvider = StreamProvider.family<
   await for (final albumRows in albumQuery.watch()) {
     // Case 1: no album row at all → cold-fetch.
     if (albumRows.isEmpty) {
-      debugPrint('albumProvider($albumId): drift miss, attempting cold-cache fetch');
       if (fetchAttempted) {
         // Already tried and got nothing back.
         yield (
@@ -245,10 +241,7 @@ final albumProvider = StreamProvider.family<
         continue;
       }
       fetchAttempted = true;
-      final online = await isOnline();
-      debugPrint('albumProvider($albumId): online=$online');
-      if (!online) {
-        debugPrint('albumProvider($albumId): offline, yielding empty');
+      if (!await isOnline()) {
         yield (
           album: const AlbumRef(id: '', title: '', artistId: ''),
           tracks: const <TrackRef>[],
@@ -265,7 +258,6 @@ final albumProvider = StreamProvider.family<
       // On success, drift watch re-emits with rows; loop continues.
       continue;
     }
-    debugPrint('albumProvider($albumId): drift hit (${albumRows.length} rows)');
 
     final albumRow = albumRows.first;
     final album = albumRow.readTable(db.cachedAlbums).toRef(
@@ -280,7 +272,6 @@ final albumProvider = StreamProvider.family<
     // fetchAndPopulate so the album becomes complete; drift watch
     // re-emits and we land in the populated branch on the next pass.
     if (trackRows.isEmpty && !fetchAttempted) {
-      debugPrint('albumProvider($albumId): album hit but no tracks; fetching');
       fetchAttempted = true;
       if (await isOnline()) {
         final ok = await fetchAndPopulate();