feat(flutter/cache): AudioCacheManager — pin/unpin/evict + tiered LRU

API:
  - isCached(trackId), pathFor(trackId)
  - pin(trackId, source) — dio.download → drift index entry
  - unpin(trackId), clearAll()
  - usageBytes(), evict(targetBytes)

Eviction order: incidental → autoPrefetch → autoPlaylist → autoLiked.
'manual' never evicts via cap-driven eviction; only clearAll() removes
operator-pinned tracks.

Two providers:
  - appDbProvider — singleton AppDb per run
  - audioCacheManagerProvider — wraps the AppDb + dioProvider

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/cache/audio_cache_manager.dart

@@ -0,0 +1,166 @@
+import 'dart:io';
+
+import 'package:dio/dio.dart';
+import 'package:drift/drift.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:path_provider/path_provider.dart';
+
+import '../library/library_providers.dart' show dioProvider;
+import 'db.dart';
+
+/// Owns the audio cache directory + drift index.
+/// API:
+///   - isCached(trackId)
+///   - pathFor(trackId)
+///   - pin(trackId, source) — downloads + indexes
+///   - unpin(trackId)
+///   - evict(targetBytes) — tiered LRU
+///   - usageBytes()
+///   - clearAll()
+class AudioCacheManager {
+  AudioCacheManager({
+    required AppDb db,
+    required Future<Dio> Function() dioFactory,
+    Future<Directory> Function()? cacheDirFactory,
+  })  : _db = db,
+        _dioFactory = dioFactory,
+        _cacheDirFactory = cacheDirFactory ?? getApplicationCacheDirectory;
+
+  final AppDb _db;
+  final Future<Dio> Function() _dioFactory;
+  final Future<Directory> Function() _cacheDirFactory;
+
+  Future<String> _tracksDir() async {
+    final base = await _cacheDirFactory();
+    final d = Directory('${base.path}/audio_cache');
+    if (!await d.exists()) await d.create(recursive: true);
+    return d.path;
+  }
+
+  /// True if the trackId has a complete file on disk.
+  Future<bool> isCached(String trackId) async {
+    final row = await (_db.select(_db.audioCacheIndex)
+          ..where((t) => t.trackId.equals(trackId)))
+        .getSingleOrNull();
+    if (row == null) return false;
+    return File(row.path).existsSync();
+  }
+
+  /// Returns the local file path if cached, else null.
+  Future<String?> pathFor(String trackId) async {
+    final row = await (_db.select(_db.audioCacheIndex)
+          ..where((t) => t.trackId.equals(trackId)))
+        .getSingleOrNull();
+    if (row == null) return null;
+    return File(row.path).existsSync() ? row.path : null;
+  }
+
+  /// Downloads the track's stream to disk and indexes it. Idempotent —
+  /// returns the existing path if already cached.
+  Future<String?> pin(String trackId, {required CacheSource source}) async {
+    final existing = await pathFor(trackId);
+    if (existing != null) return existing;
+
+    final dir = await _tracksDir();
+    final path = '$dir/$trackId.mp3';
+    final dio = await _dioFactory();
+    try {
+      await dio.download('/api/tracks/$trackId/stream', path);
+    } catch (_) {
+      final f = File(path);
+      if (f.existsSync()) await f.delete();
+      return null;
+    }
+    final size = await File(path).length();
+    await _db.into(_db.audioCacheIndex).insertOnConflictUpdate(
+          AudioCacheIndexCompanion.insert(
+            trackId: trackId,
+            path: path,
+            sizeBytes: size,
+            source: source,
+          ),
+        );
+    return path;
+  }
+
+  /// Removes a track from the index AND deletes the file.
+  Future<void> unpin(String trackId) async {
+    final row = await (_db.select(_db.audioCacheIndex)
+          ..where((t) => t.trackId.equals(trackId)))
+        .getSingleOrNull();
+    if (row == null) return;
+    final f = File(row.path);
+    if (f.existsSync()) await f.delete();
+    await (_db.delete(_db.audioCacheIndex)
+          ..where((t) => t.trackId.equals(trackId)))
+        .go();
+  }
+
+  /// Total bytes used by the cache.
+  Future<int> usageBytes() async {
+    final result = await _db.customSelect(
+      'SELECT COALESCE(SUM(size_bytes), 0) AS total FROM audio_cache_index',
+      readsFrom: {_db.audioCacheIndex},
+    ).getSingle();
+    return result.read<int>('total');
+  }
+
+  /// Evicts files until usage ≤ targetBytes. Eviction order:
+  /// incidental → autoPrefetch → autoPlaylist → autoLiked.
+  /// `manual` never evicts via this path; only clearAll() removes them.
+  Future<void> evict({required int targetBytes}) async {
+    final used = await usageBytes();
+    if (used <= targetBytes) return;
+    final order = [
+      CacheSource.incidental,
+      CacheSource.autoPrefetch,
+      CacheSource.autoPlaylist,
+      CacheSource.autoLiked,
+    ];
+    int remaining = used - targetBytes;
+    for (final src in order) {
+      if (remaining <= 0) break;
+      final rows = await (_db.select(_db.audioCacheIndex)
+            ..where((t) => t.source.equalsValue(src))
+            ..orderBy([(t) => OrderingTerm.asc(t.cachedAt)]))
+          .get();
+      for (final row in rows) {
+        if (remaining <= 0) break;
+        final f = File(row.path);
+        if (f.existsSync()) await f.delete();
+        await (_db.delete(_db.audioCacheIndex)
+              ..where((t) => t.trackId.equals(row.trackId)))
+            .go();
+        remaining -= row.sizeBytes;
+      }
+    }
+  }
+
+  /// Clears EVERY row + EVERY file. Wired to the operator's "Clear cache"
+  /// button — `manual` source rows are removed here but only here.
+  Future<void> clearAll() async {
+    final dir = await _tracksDir();
+    final d = Directory(dir);
+    if (d.existsSync()) {
+      for (final entity in d.listSync()) {
+        if (entity is File) await entity.delete();
+      }
+    }
+    await _db.delete(_db.audioCacheIndex).go();
+  }
+}
+
+/// AppDb singleton. One per app run; ref.onDispose closes it.
+final appDbProvider = Provider<AppDb>((ref) {
+  final db = AppDb();
+  ref.onDispose(db.close);
+  return db;
+});
+
+final audioCacheManagerProvider = Provider<AudioCacheManager>((ref) {
+  final db = ref.watch(appDbProvider);
+  return AudioCacheManager(
+    db: db,
+    dioFactory: () async => ref.read(dioProvider.future),
+  );
+});