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>

web/src/routes/playlists/playlists.test.ts

@@ -19,7 +19,7 @@ const mockedCreatePlaylist = createPlaylist as ReturnType<typeof vi.fn>;
 function p(over: Partial<Playlist>): Playlist {
   return {
     id: 'p1', user_id: 'u-self', owner_username: 'me', name: 'A',
-    description: '', is_public: false, kind: 'user', system_variant: null, seed_artist_id: null, cover_url: '', track_count: 0, duration_sec: 0,
+    description: '', is_public: false, kind: 'user', system_variant: null, refreshable: false, seed_artist_id: null, cover_url: '', track_count: 0, duration_sec: 0,
     created_at: '', updated_at: '', ...over
   };
 }