feat(library): add bulk duplicate cleanup

lib/utils/duplicate_cleanup_policy.dart

@@ -0,0 +1,39 @@
+import 'package:spotiflac_android/services/library_database.dart';
+
+class DuplicateCleanupPlan {
+  final List<IsrcDuplicateEntry> retainedEntries;
+  final List<IsrcDuplicateEntry> entriesToDelete;
+  final int groupCount;
+
+  const DuplicateCleanupPlan({
+    required this.retainedEntries,
+    required this.entriesToDelete,
+    required this.groupCount,
+  });
+}
+
+/// Keeps the first (highest-quality) entry from every duplicate group and
+/// schedules every remaining copy for deletion.
+///
+/// [LibraryDatabase.findIsrcDuplicateGroups] returns entries best-quality
+/// first, so this policy deliberately preserves that ordering contract.
+DuplicateCleanupPlan buildKeepBestAllDuplicatePlan(
+  List<IsrcDuplicateGroup> groups,
+) {
+  final retainedEntries = <IsrcDuplicateEntry>[];
+  final entriesToDelete = <IsrcDuplicateEntry>[];
+  var groupCount = 0;
+
+  for (final group in groups) {
+    if (group.entries.length < 2) continue;
+    groupCount++;
+    retainedEntries.add(group.entries.first);
+    entriesToDelete.addAll(group.entries.skip(1));
+  }
+
+  return DuplicateCleanupPlan(
+    retainedEntries: List.unmodifiable(retainedEntries),
+    entriesToDelete: List.unmodifiable(entriesToDelete),
+    groupCount: groupCount,
+  );
+}