feat(library): add bulk duplicate cleanup

test/duplicate_cleanup_policy_test.dart

@@ -0,0 +1,49 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/library_database.dart';
+import 'package:spotiflac_android/utils/duplicate_cleanup_policy.dart';
+
+IsrcDuplicateEntry _entry(String id) {
+  return IsrcDuplicateEntry(
+    id: id,
+    source: 'local',
+    trackName: 'Track $id',
+    artistName: 'Artist',
+    albumName: 'Album',
+    filePath: '/music/$id.flac',
+  );
+}
+
+void main() {
+  test('keep-best-all retains one entry and deletes the rest per group', () {
+    final firstBest = _entry('first-best');
+    final firstLower = _entry('first-lower');
+    final firstLowest = _entry('first-lowest');
+    final secondBest = _entry('second-best');
+    final secondLower = _entry('second-lower');
+
+    final plan = buildKeepBestAllDuplicatePlan([
+      IsrcDuplicateGroup(
+        isrc: 'FIRST',
+        entries: [firstBest, firstLower, firstLowest],
+      ),
+      IsrcDuplicateGroup(isrc: 'SECOND', entries: [secondBest, secondLower]),
+    ]);
+
+    expect(plan.groupCount, 2);
+    expect(plan.retainedEntries, [firstBest, secondBest]);
+    expect(plan.entriesToDelete, [firstLower, firstLowest, secondLower]);
+  });
+
+  test('keep-best-all ignores groups that no longer contain duplicates', () {
+    final onlyCopy = _entry('only-copy');
+
+    final plan = buildKeepBestAllDuplicatePlan([
+      IsrcDuplicateGroup(isrc: 'ONLY', entries: [onlyCopy]),
+      const IsrcDuplicateGroup(isrc: 'EMPTY', entries: []),
+    ]);
+
+    expect(plan.groupCount, 0);
+    expect(plan.retainedEntries, isEmpty);
+    expect(plan.entriesToDelete, isEmpty);
+  });
+}