feat(flutter): SystemPlaylistsStatus model + endpoint method

Adds the model + MeApi.systemPlaylistsStatus() for the home Playlists
row's placeholder-variant logic (building / failed / pending /
seed-needed). Mirrors GET /api/me/system-playlists-status which
returns the caller's most recent system_playlist_runs row.

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

flutter_client/lib/models/system_playlists_status.dart

@@ -0,0 +1,24 @@
+/// Mirrors `systemPlaylistsStatusResp` from internal/api/me_system_playlists.go.
+/// Reflects the caller's most recent system_playlist_runs row, or zero
+/// values when no row exists yet (the user has never had a build attempted).
+class SystemPlaylistsStatus {
+  const SystemPlaylistsStatus({
+    required this.inFlight,
+    this.lastRunAt,
+    this.lastError,
+  });
+
+  final bool inFlight;
+  final String? lastRunAt;
+  final String? lastError;
+
+  factory SystemPlaylistsStatus.empty() =>
+      const SystemPlaylistsStatus(inFlight: false);
+
+  factory SystemPlaylistsStatus.fromJson(Map<String, dynamic> j) =>
+      SystemPlaylistsStatus(
+        inFlight: j['in_flight'] as bool? ?? false,
+        lastRunAt: j['last_run_at'] as String?,
+        lastError: j['last_error'] as String?,
+      );
+}