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/playlists/playlist_detail_screen.dart

@@ -212,7 +212,7 @@ class _Body extends ConsumerWidget {
                     ref.read(playerActionsProvider).playTracks(
                           playableRefs,
                           initialIndex: startIdx >= 0 ? startIdx : 0,
-                          source: detail.playlist.isSystem
+                          source: detail.playlist.refreshable
                               ? detail.playlist.systemVariant
                               : null,
                         );
@@ -266,7 +266,7 @@ class _Header extends ConsumerWidget {
           ),
           const Spacer(),
           if (playable.isNotEmpty) ...[
-            if (p.isSystem)
+            if (p.refreshable)
               OutlinedButton.icon(
                 key: const Key('regenerate_playlist_button'),
                 onPressed: () => _regenerate(context, ref),
@@ -298,7 +298,7 @@ class _Header extends ConsumerWidget {
                 final refs = playable.map(_toTrackRef).toList(growable: false);
                 ref.read(playerActionsProvider).playTracks(
                       refs,
-                      source: p.isSystem ? p.systemVariant : null,
+                      source: p.refreshable ? p.systemVariant : null,
                     );
               },
               icon: const Icon(Icons.play_arrow),

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -69,7 +69,7 @@ class PlaylistCard extends ConsumerWidget {
                   // System playlists get a refresh affordance so the
                   // user can force a fresh mix instead of waiting for
                   // the daily 03:00 rebuild. Mirrors the web kebab.
-                  if (playlist.isSystem)
+                  if (playlist.refreshable)
                     Positioned(
                       top: 2,
                       right: 2,
@@ -123,11 +123,7 @@ class PlaylistCard extends ConsumerWidget {
     );
   }
 
-  String get _refreshLabel => switch (playlist.systemVariant) {
-        'for_you' => 'Refresh For You',
-        'discover' => 'Refresh Discover',
-        _ => 'Refresh',
-      };
+  String get _refreshLabel => 'Refresh ${playlist.name}';
 
   /// Forces a server-side rebuild of this system playlist, then
   /// invalidates the aggregate list so the rotated UUID + new tracks
@@ -155,11 +151,12 @@ class PlaylistCard extends ConsumerWidget {
   /// index 0. Mirrors the web PlaylistCard's onPlayClick.
   Future<void> _playPlaylist(WidgetRef ref) async {
     final api = await ref.read(playlistsApiProvider.future);
-    // System playlists: fetch the server's rotation-aware order
-    // (#415) and play it as-is, tagged with the source so the
-    // play-events reporter advances that playlist's rotation. User
-    // playlists keep stored order, untagged.
-    final detail = playlist.isSystem
+    // Refreshable (singleton) system playlists: fetch the server's
+    // rotation-aware order (#415) and play as-is, tagged with the
+    // source so the reporter advances rotation. User playlists AND
+    // non-singleton system kinds (songs_like_artist — no by-kind
+    // endpoint) play the stored order via get(), untagged.
+    final detail = playlist.refreshable
         ? await api.systemShuffle(playlist.systemVariant!)
         : await api.get(playlist.id);
     final refs = <TrackRef>[];
@@ -177,14 +174,13 @@ class PlaylistCard extends ConsumerWidget {
       ));
     }
     if (refs.isEmpty) return;
-    // System playlists already arrive in rotation-aware order from
-    // the server (#415) — play as-is, tagged with the variant so the
-    // reporter advances rotation. No client shuffle. User playlists
-    // play in stored order, untagged.
+    // Rotation-aware kinds arrive pre-ordered from the server — play
+    // as-is, tagged so the reporter advances rotation. No client
+    // shuffle. Everything else plays stored order, untagged.
     await ref.read(playerActionsProvider).playTracks(
           refs,
           initialIndex: 0,
-          source: playlist.isSystem ? playlist.systemVariant : null,
+          source: playlist.refreshable ? playlist.systemVariant : null,
         );
   }
 }