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/playlists/playlists_provider.dart

@@ -40,11 +40,6 @@ final playlistsListProvider =
     fetchAndPopulate: () async {
       final api = await ref.read(playlistsApiProvider.future);
       final fresh = await api.list(kind: kind);
-      final ownedSysCount =
-          fresh.owned.where((p) => p.systemVariant != null).length;
-      debugPrint(
-          'playlistsListProvider($kind): wire returned owned=${fresh.owned.length} '
-          '(system=$ownedSysCount) public=${fresh.public.length}');
 
       // Reconcile: BuildSystemPlaylists rotates system-playlist UUIDs
       // every rebuild, so insertOrReplace alone leaves stale rows in
@@ -81,13 +76,6 @@ final playlistsListProvider =
           .where((r) => r.userId != user.id && r.isPublic)
           .map((r) => r.toRef())
           .toList();
-      final ownedSys = owned.where((p) => p.isSystem).length;
-      final driftSys = filtered.where((r) => r.systemVariant != null).length;
-      debugPrint(
-          'playlistsListProvider($kind): drift rows=${rows.length} '
-          'filtered=${filtered.length} (system=$driftSys) '
-          'owned=${owned.length} (system=$ownedSys) pub=${pub.length} '
-          'userId=${user.id}');
       return PlaylistsList(owned: owned, public: pub);
     },
     isOnline: () async => (await ref
@@ -153,10 +141,7 @@ final playlistDetailProvider =
   Future<bool> fetchAndPopulate() async {
     try {
       final api = await ref.read(playlistsApiProvider.future);
-      debugPrint('playlistDetailProvider($id): calling get');
       final fresh = await api.get(id).timeout(const Duration(seconds: 10));
-      debugPrint(
-          'playlistDetailProvider($id): got ${fresh.tracks.length} tracks, writing');
 
       // Collect the track + artist + album rows referenced by these
       // playlist entries. Without writing the cachedTracks rows
@@ -262,8 +247,6 @@ final playlistDetailProvider =
           );
         }
       });
-      debugPrint(
-          'playlistDetailProvider($id): drift write done; awaiting watch re-emit');
       return true;
     } catch (e, st) {
       // 404 = the playlist row in drift is stale (BuildSystemPlaylists
@@ -272,8 +255,6 @@ final playlistDetailProvider =
       // stale row + its track positions so the home tile disappears
       // on next render and we don't keep trying.
       if (e is DioException && e.response?.statusCode == 404) {
-        debugPrint(
-            'playlistDetailProvider($id): server says 404, evicting stale drift rows');
         await db.batch((b) {
           b.deleteWhere(
               db.cachedPlaylistTracks, (t) => t.playlistId.equals(id));
@@ -292,7 +273,6 @@ final playlistDetailProvider =
 
   await for (final playlistRows in playlistQuery.watch()) {
     if (playlistRows.isEmpty) {
-      debugPrint('playlistDetailProvider($id): drift miss, cold-fetching');
       if (fetchAttempted) {
         yield emptyDetail();
         continue;
@@ -307,9 +287,6 @@ final playlistDetailProvider =
       continue;
     }
 
-    debugPrint(
-        'playlistDetailProvider($id): drift hit (${playlistRows.length} rows)');
-
     final playlist = playlistRows.first.toRef();
     final trackRows = await tracksQuery.get();
 
@@ -318,8 +295,6 @@ final playlistDetailProvider =
     // tracks for system playlists). Trigger the same fetch, drift
     // watch re-emits with populated tracks.
     if (trackRows.isEmpty && !fetchAttempted) {
-      debugPrint(
-          'playlistDetailProvider($id): playlist hit but no tracks; fetching');
       fetchAttempted = true;
       if (await isOnline()) {
         final ok = await fetchAndPopulate();