refactor(playlists): #411 R2 — generic registry-driven system endpoints

Replaces the per-kind refresh/shuffle handlers with one generic
pair driven off the kind registry, in lockstep across both clients.

Server:
- systemPlaylistKind gains Singleton; RefreshableSystemKind(key)
  exported. for_you/discover singleton; songs_like_artist not.
- New generic POST /api/playlists/system/{kind}/refresh and
  GET /api/playlists/system/{kind}/shuffle ({kind} = raw
  system_variant). Non-singleton/unknown kind → 404. Deleted
  playlists_{foryou,discover}_refresh.go and the per-kind shuffle
  wrappers; serveSystemPlaylistShuffle core kept.
- playlistRowView.refreshable: server-derived flag so clients show
  the refresh affordance generically without hardcoding kinds.

Web (not drift-cached → uses the server flag):
- refreshSystem(variant) replaces refreshForYou/refreshDiscover;
  systemShuffle drops the for_you→for-you mapping (raw variant).
- PlaylistCard + detail page gate the kebab/Refresh button on
  playlist.refreshable; label is "Refresh {name}". Tests reworked;
  obsolete refresh-foryou/discover api tests deleted.

Flutter (list tiles are drift-cache-sourced → derive, no migration):
- Playlist.refreshable getter = isSystem && variant !=
  songs_like_artist (holds for all current + planned kinds).
- refreshSystem/systemShuffle use the raw variant; PlaylistCard +
  detail screen gate kebab/Regenerate/source-tagging on refreshable
  so songs_like_artist plays via get() (no by-kind endpoint).

Pure-plumbing refactor; CI verifies parity. Next (R3): the five
discovery mixes — each a candidate query + one registry entry.

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

flutter_client/lib/models/playlist.dart

@@ -38,6 +38,16 @@ class Playlist {
 
   bool get isSystem => systemVariant != null;
 
+  /// Whether this playlist supports the generic by-kind refresh/
+  /// shuffle endpoints (#411 R2) — i.e. a singleton system kind.
+  /// The server exposes a `refreshable` flag for JSON-sourced
+  /// playlists, but the list tiles are drift-cache-sourced (no
+  /// migration just for this), so derive it: every system kind is a
+  /// singleton except songs_like_artist (multi-per-user). This rule
+  /// holds for For-You/Discover and all planned discovery mixes; if
+  /// a future non-singleton kind appears, extend the exclusion.
+  bool get refreshable => isSystem && systemVariant != 'songs_like_artist';
+
   factory Playlist.fromJson(Map<String, dynamic> j) => Playlist(
         id: j['id'] as String,
         userId: j['user_id'] as String? ?? '',