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/PlaylistCard.svelte

@@ -3,7 +3,7 @@
   import { useQueryClient } from '@tanstack/svelte-query';
   import type { Playlist, PlaylistTrack, TrackRef } from '$lib/api/types';
   import { user } from '$lib/auth/store.svelte';
-  import { getPlaylist, systemShuffle, refreshDiscover, refreshForYou } from '$lib/api/playlists';
+  import { getPlaylist, systemShuffle, refreshSystem } from '$lib/api/playlists';
   import { playlistTrackToRef } from '$lib/playlists/playlistTrackToRef';
   import { errCode } from '$lib/api/errors';
   import { qk } from '$lib/api/queries';
@@ -13,12 +13,9 @@
   let { playlist }: { playlist: Playlist } = $props();
 
   const isOwn = $derived(user.value?.id === playlist.user_id);
-  const isDiscover = $derived(playlist.system_variant === 'discover');
-  const isForYou = $derived(playlist.system_variant === 'for_you');
-  const isSystem = $derived(playlist.system_variant != null);
-  const refreshLabel = $derived(
-    isDiscover ? 'Refresh Discover' : isForYou ? 'Refresh For You' : 'Refresh',
-  );
+  // Server tells us which system playlists support by-kind refresh
+  // (#411 R2) — no hardcoded kind list here.
+  const refreshable = $derived(playlist.refreshable === true);
 
   const queryClient = useQueryClient();
 
@@ -72,16 +69,10 @@
     e.preventDefault();
     e.stopPropagation();
     menuOpen = false;
+    if (!refreshable || playlist.system_variant == null) return;
     try {
-      if (isDiscover) {
-        await refreshDiscover();
-        pushToast('Discover refreshed.');
-      } else if (isForYou) {
-        await refreshForYou();
-        pushToast('For You refreshed.');
-      } else {
-        return;
-      }
+      await refreshSystem(playlist.system_variant);
+      pushToast(`${playlist.name} refreshed.`);
       await queryClient.invalidateQueries({ queryKey: qk.playlist(playlist.id) });
       await queryClient.invalidateQueries({ queryKey: qk.playlists() });
     } catch (err: unknown) {
@@ -93,7 +84,7 @@
 <svelte:window onclick={() => { menuOpen = false; }} />
 
 <div class="card relative">
-  {#if isSystem}
+  {#if refreshable}
     <div class="kebab-wrap absolute right-1 top-1 z-10">
       <button
         type="button"
@@ -122,7 +113,7 @@
             class="flex w-full items-center gap-2 rounded px-2 py-1.5 text-sm text-text-primary hover:bg-surface-hover"
           >
             <RefreshCcw size={14} strokeWidth={1} />
-            {refreshLabel}
+            Refresh {playlist.name}
           </button>
         </div>
       {/if}