feat(flutter): admin DTOs (request, quarantine item, user, invite)

Hand-rolled to match the server's actual JSON shapes:
- AdminRequest carries user_id (UUID, not username); kind switches
  which title field is the display name
- AdminQuarantineItem aggregates reports per track with reason_counts
  and a nested reports[] list, mirroring adminQueueRowView
- AdminUser includes display_name + auto_approve_requests
- Invite uses invited_by UUID + note (server hardcodes 24h TTL)

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

flutter_client/lib/models/admin_request.dart

@@ -0,0 +1,64 @@
+/// Mirrors `requestView` from internal/api/requests.go. Server returns
+/// these as a flat list (no envelope) from GET /api/admin/requests.
+///
+/// Note the requester is exposed only as a UUID (`user_id`) — the
+/// response does not include the username. The Flutter Requests screen
+/// joins client-side against the AdminUser list for display.
+class AdminRequest {
+  const AdminRequest({
+    required this.id,
+    required this.userId,
+    required this.status,
+    required this.kind,
+    required this.artistName,
+    this.albumTitle,
+    this.trackTitle,
+    required this.requestedAt,
+    this.decidedAt,
+    this.notes,
+    required this.importedAlbumCount,
+    required this.importedTrackCount,
+  });
+
+  final String id;
+  final String userId;
+
+  /// One of: pending, approved, rejected, completed, failed.
+  final String status;
+
+  /// One of: artist, album, track.
+  final String kind;
+
+  final String artistName;
+  final String? albumTitle;
+  final String? trackTitle;
+  final String requestedAt;
+  final String? decidedAt;
+  final String? notes;
+  final int importedAlbumCount;
+  final int importedTrackCount;
+
+  /// Display label depending on the kind of request — what the user
+  /// asked for. For an album request it's the album title; for an
+  /// artist request it's the artist name; etc.
+  String get displayName => switch (kind) {
+        'album' => albumTitle ?? artistName,
+        'track' => trackTitle ?? artistName,
+        _ => artistName,
+      };
+
+  factory AdminRequest.fromJson(Map<String, dynamic> j) => AdminRequest(
+        id: j['id'] as String? ?? '',
+        userId: j['user_id'] as String? ?? '',
+        status: j['status'] as String? ?? 'pending',
+        kind: j['kind'] as String? ?? 'artist',
+        artistName: j['artist_name'] as String? ?? '',
+        albumTitle: j['album_title'] as String?,
+        trackTitle: j['track_title'] as String?,
+        requestedAt: j['requested_at'] as String? ?? '',
+        decidedAt: j['decided_at'] as String?,
+        notes: j['notes'] as String?,
+        importedAlbumCount: (j['imported_album_count'] as int?) ?? 0,
+        importedTrackCount: (j['imported_track_count'] as int?) ?? 0,
+      );
+}