fix(flutter): drop unused imports flagged by analyze

Slice A landed with three transitive imports that the analyzer
correctly flagged as unused. AppDb / CachedAlbums table refs
propagate through audio_cache_manager.dart's `show appDbProvider`
re-export chain so the explicit db.dart import in the consumers
is redundant.

flutter_client/lib/cache/hydration_queue.dart

@@ -22,10 +22,8 @@ import 'dart:collection';
 
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
-import '../api/endpoints/library.dart';
 import '../cache/adapters.dart';
 import '../cache/audio_cache_manager.dart' show appDbProvider;
-import '../cache/db.dart';
 import '../library/library_providers.dart' show libraryApiProvider;
 
 /// Bounded-concurrency request queue for per-entity hydration. One

flutter_client/lib/cache/tile_providers.dart

@@ -23,7 +23,6 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../cache/adapters.dart';
 import '../cache/audio_cache_manager.dart' show appDbProvider;
-import '../cache/db.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
 import '../models/track.dart';