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/models/lidarr.dart

@@ -0,0 +1,47 @@
+/// Mirrors web/src/lib/api/types.ts LidarrSearchResult — one row in
+/// `/api/lidarr/search` results. `in_library` and `requested` let the
+/// UI gray out rows the user can't act on (already imported / already
+/// awaiting review).
+class LidarrSearchResult {
+  const LidarrSearchResult({
+    required this.mbid,
+    required this.name,
+    required this.secondaryText,
+    required this.imageUrl,
+    required this.artistMbid,
+    required this.albumMbid,
+    required this.inLibrary,
+    required this.requested,
+  });
+
+  final String mbid;
+  final String name;
+  final String secondaryText;
+  final String imageUrl;
+  final String artistMbid;
+  final String albumMbid;
+  final bool inLibrary;
+  final bool requested;
+
+  factory LidarrSearchResult.fromJson(Map<String, dynamic> j) =>
+      LidarrSearchResult(
+        mbid: j['mbid'] as String? ?? '',
+        name: j['name'] as String? ?? '',
+        secondaryText: j['secondary_text'] as String? ?? '',
+        imageUrl: j['image_url'] as String? ?? '',
+        artistMbid: j['artist_mbid'] as String? ?? '',
+        albumMbid: j['album_mbid'] as String? ?? '',
+        inLibrary: j['in_library'] as bool? ?? false,
+        requested: j['requested'] as bool? ?? false,
+      );
+}
+
+enum LidarrRequestKind { artist, album, track }
+
+extension LidarrRequestKindStr on LidarrRequestKind {
+  String get wire => switch (this) {
+        LidarrRequestKind.artist => 'artist',
+        LidarrRequestKind.album => 'album',
+        LidarrRequestKind.track => 'track',
+      };
+}