feat(flutter): discover screen — search Lidarr + create requests

- models/lidarr.dart (LidarrSearchResult + LidarrRequestKind)
- api/endpoints/discover.dart (search + createRequest)
- discover/discover_screen.dart with TextField, kind toggle (Artists/Albums),
  results list with in_library / requested pills
- /discover route + explore icon on home AppBar (5 actions now: Library,
  Playlists, Search, Discover, Settings)

flutter_client/lib/api/endpoints/discover.dart

@@ -0,0 +1,59 @@
+import 'package:dio/dio.dart';
+
+import '../../models/lidarr.dart';
+
+class DiscoverApi {
+  DiscoverApi(this._dio);
+  final Dio _dio;
+
+  /// GET /api/lidarr/search?q=...&kind=artist|album|track. Server has a
+  /// 60s LRU cache for repeat queries so re-typing the same string in
+  /// quick succession is cheap.
+  Future<List<LidarrSearchResult>> search(
+    String query,
+    LidarrRequestKind kind,
+  ) async {
+    final r = await _dio.get<List<dynamic>>(
+      '/api/lidarr/search',
+      queryParameters: {'q': query, 'kind': kind.wire},
+    );
+    final raw = r.data ?? const [];
+    return raw
+        .map((e) =>
+            LidarrSearchResult.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+
+  /// POST /api/requests. Returns the newly-created LidarrRequest body
+  /// — we don't expose a typed wrapper for that yet on mobile (admin
+  /// reviews requests; user just kicks them off), so we discard the
+  /// response and surface success/failure to the caller.
+  Future<void> createRequest({
+    required LidarrRequestKind kind,
+    required String artistMbid,
+    required String artistName,
+    String? albumMbid,
+    String? albumTitle,
+    String? trackMbid,
+    String? trackTitle,
+  }) async {
+    final body = <String, dynamic>{
+      'kind': kind.wire,
+      'lidarr_artist_mbid': artistMbid,
+      'artist_name': artistName,
+    };
+    if (albumMbid != null && albumMbid.isNotEmpty) {
+      body['lidarr_album_mbid'] = albumMbid;
+    }
+    if (trackMbid != null && trackMbid.isNotEmpty) {
+      body['lidarr_track_mbid'] = trackMbid;
+    }
+    if (albumTitle != null && albumTitle.isNotEmpty) {
+      body['album_title'] = albumTitle;
+    }
+    if (trackTitle != null && trackTitle.isNotEmpty) {
+      body['track_title'] = trackTitle;
+    }
+    await _dio.post<Map<String, dynamic>>('/api/requests', data: body);
+  }
+}