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/lib/components/AddToPlaylistMenu.test.ts

@@ -14,6 +14,7 @@ const playlistsData = vi.hoisted(() => ({
       is_public: false,
       kind: 'user',
       system_variant: null,
+      refreshable: false,
       seed_artist_id: null,
       cover_url: '',
       track_count: 3,
@@ -30,6 +31,7 @@ const playlistsData = vi.hoisted(() => ({
       is_public: false,
       kind: 'user',
       system_variant: null,
+      refreshable: false,
       seed_artist_id: null,
       cover_url: '',
       track_count: 5,
@@ -58,6 +60,7 @@ vi.mock('$lib/api/playlists', () => ({
     is_public: false,
     kind: 'user',
     system_variant: null,
+    refreshable: false,
     seed_artist_id: null,
     cover_url: '',
     track_count: 0,