feat(flutter/api): QuarantineApi for flag/unflag

Dio client for /api/quarantine — flag a track with a reason + optional
notes, unflag by track id. Mirrors the server's user-scoped quarantine
endpoints (separate from /admin/quarantine).

Used by the track-actions menu's Hide/Unhide action in the next slice
of commits.

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

flutter_client/lib/api/endpoints/quarantine.dart

@@ -0,0 +1,25 @@
+import 'package:dio/dio.dart';
+
+/// /api/quarantine — flag a track (with reason + optional notes) and
+/// unflag it. Both endpoints are user-scoped: callers can only flag
+/// their own quarantine entries; admins use a separate /admin/quarantine
+/// surface.
+class QuarantineApi {
+  QuarantineApi(this._dio);
+  final Dio _dio;
+
+  /// POST /api/quarantine. Server returns 201 with the row.
+  /// Reason values: bad_rip | wrong_file | wrong_tags | duplicate | other.
+  Future<void> flag(String trackId, String reason, {String notes = ''}) async {
+    await _dio.post<void>('/api/quarantine', data: {
+      'track_id': trackId,
+      'reason': reason,
+      'notes': notes,
+    });
+  }
+
+  /// DELETE /api/quarantine/{track_id}. Server returns 204.
+  Future<void> unflag(String trackId) async {
+    await _dio.delete<void>('/api/quarantine/$trackId');
+  }
+}