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),