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/cache/cache_first.dart

@@ -19,6 +19,10 @@ import 'dart:async';
 
 import 'package:flutter/foundation.dart' show debugPrint;
 
+// `tag` parameter is preserved for future ad-hoc instrumentation.
+// Normal operation only logs failure paths so the per-screen log
+// noise stays low.
+
 /// Wraps the watch + cold-cache fallback pattern. Generic over:
 ///   D — the drift row type (or TypedResult for joins)
 ///   T — the result type the caller wants (e.g. `List<ArtistRef>`)
@@ -43,13 +47,9 @@ Stream<T> cacheFirst<D, T>({
   // refresh for this stream subscription, so we don't fire one on every
   // drift re-emission (otherwise the populate cycles forever).
   var revalidated = false;
-  void log(String msg) {
-    if (tag != null) debugPrint('cacheFirst[$tag]: $msg');
-  }
 
   await for (final rows in driftStream) {
     if (rows.isNotEmpty) {
-      log('drift hit (${rows.length} rows)');
       yield toResult(rows);
       // Stale-while-revalidate: yield cache immediately, then kick off
       // a REST refresh in the background. Drift watch() picks up the
@@ -62,20 +62,18 @@ Stream<T> cacheFirst<D, T>({
       }
       continue;
     }
-    log('drift miss; checking connectivity');
     if (await isOnline()) {
-      log('online; calling fetchAndPopulate');
       try {
         await fetchAndPopulate();
-        log('fetchAndPopulate done; awaiting drift re-emit');
         // The drift watch() stream re-emits with the populated rows on
         // the next loop iteration. Don't yield here.
       } catch (e, st) {
-        log('fetchAndPopulate failed: $e\n$st');
+        if (tag != null) {
+          debugPrint('cacheFirst[$tag]: fetchAndPopulate failed: $e\n$st');
+        }
         yield toResult(rows); // empty result; caller surfaces error
       }
     } else {
-      log('offline; yielding empty');
       yield toResult(rows); // empty result; offline
     }
   }

flutter_client/lib/cache/metadata_prefetcher.dart

@@ -1,4 +1,3 @@
-import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../library/library_providers.dart';
@@ -38,7 +37,6 @@ class MetadataPrefetcher {
       if (n++ >= _topN) break;
       _swallow(_ref.read(artistProvider(id).future));
     }
-    if (n > 0) debugPrint('metadataPrefetcher: warming $n artists');
   }
 
   void _warmHome(HomeData h) {