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/player/player_provider.dart

@@ -1,5 +1,4 @@
 import 'package:audio_service/audio_service.dart';
-import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/radio.dart';
@@ -54,22 +53,8 @@ class PlayerActions {
   final Ref _ref;
 
   Future<void> playTracks(List<TrackRef> tracks, {int initialIndex = 0}) async {
-    // Stage-by-stage timing so we can see exactly where the
-    // "tap → audio" delay lives. Look for "playTracks: ..." lines
-    // in the next log capture; each stage label is followed by its
-    // elapsed millis since the previous stage.
-    final sw = Stopwatch()..start();
-    int lap() {
-      final ms = sw.elapsedMilliseconds;
-      sw.reset();
-      sw.start();
-      return ms;
-    }
-
     final url   = await _ref.read(serverUrlProvider.future);
-    debugPrint('playTracks: serverUrl ${lap()}ms');
     final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
-    debugPrint('playTracks: token ${lap()}ms');
     final cache = _ref.read(albumCoverCacheProvider);
     final audioCache = _ref.read(audioCacheManagerProvider);
     final h = _ref.read(audioHandlerProvider)
@@ -79,11 +64,8 @@ class PlayerActions {
         coverCache: cache,
         audioCacheManager: audioCache,
       );
-    debugPrint('playTracks: configure ${lap()}ms');
     await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
-    debugPrint('playTracks: setQueue (${tracks.length} tracks) ${lap()}ms');
     await h.play();
-    debugPrint('playTracks: play() returned ${lap()}ms');
   }
 
   Future<void> playNext(TrackRef track) async {