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/api/endpoints/playlists.dart

@@ -45,15 +45,13 @@ class PlaylistsApi {
     return PlaylistDetail.fromJson(r.data ?? const {});
   }
 
-  /// GET /api/playlists/system/{variant}/shuffle (#415 stage 2/3).
+  /// GET /api/playlists/system/{kind}/shuffle (#415 / #411 R2).
   /// Same shape as get() but tracks are server-ordered rotation-aware
-  /// (unplayed-this-rotation first; resets when exhausted). Model
-  /// variant uses underscores; the route segment is hyphenated.
-  /// Intentionally uncached — varies per play.
+  /// (unplayed-this-rotation first; resets when exhausted). {kind} is
+  /// the raw system_variant. Intentionally uncached — varies per play.
   Future<PlaylistDetail> systemShuffle(String variant) async {
-    final seg = variant == 'for_you' ? 'for-you' : variant;
     final r = await _dio.get<Map<String, dynamic>>(
-      '/api/playlists/system/$seg/shuffle',
+      '/api/playlists/system/$variant/shuffle',
     );
     return PlaylistDetail.fromJson(r.data ?? const {});
   }
@@ -67,17 +65,14 @@ class PlaylistsApi {
     );
   }
 
-  /// POST /api/playlists/system/{variant}/refresh. Synchronously
-  /// rebuilds the caller's system playlist for the given variant
-  /// ("for_you" | "discover") and returns the new playlist id, or
-  /// null when the library is empty so there's nothing to build.
-  ///
-  /// The variant uses underscores in the model (system_variant) but
-  /// the route segment is hyphenated ("for-you"), so map here.
+  /// POST /api/playlists/system/{kind}/refresh (#411 R2). Rebuilds
+  /// the caller's system playlists and returns the named kind's new
+  /// playlist id, or null when the library is empty. {kind} is the
+  /// raw system_variant — the server routes generically off the
+  /// kind registry, no hyphen mapping.
   Future<String?> refreshSystem(String variant) async {
-    final segment = variant == 'for_you' ? 'for-you' : variant;
     final r = await _dio.post<Map<String, dynamic>>(
-      '/api/playlists/system/$segment/refresh',
+      '/api/playlists/system/$variant/refresh',
       data: const <String, dynamic>{},
     );
     return (r.data ?? const {})['playlist_id'] as String?;