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/[id]/+page.svelte

@@ -12,7 +12,7 @@
     deletePlaylist,
     removePlaylistTrack,
     reorderPlaylist,
-    refreshDiscover
+    refreshSystem
   } from '$lib/api/playlists';
   import { qk } from '$lib/api/queries';
   import { user } from '$lib/auth/store.svelte';
@@ -130,20 +130,20 @@
     }
   }
 
-  // --- Discover refresh ---
-  let refreshingDiscover = $state(false);
+  // --- System-playlist refresh (#411 R2: generic by-kind) ---
+  let refreshingSystem = $state(false);
 
-  async function onRefreshDiscover() {
-    refreshingDiscover = true;
+  async function onRefreshSystem(variant: string) {
+    refreshingSystem = true;
     try {
-      await refreshDiscover();
-      pushToast('Discover refreshed.');
+      await refreshSystem(variant);
+      pushToast('Refreshed.');
       await queryClient.invalidateQueries({ queryKey: qk.playlist(id) });
       await queryClient.invalidateQueries({ queryKey: qk.playlists() });
     } catch (e: unknown) {
       pushToast(`Refresh failed: ${errCode(e)}`, 'error');
     } finally {
-      refreshingDiscover = false;
+      refreshingSystem = false;
     }
   }
 </script>
@@ -177,15 +177,15 @@
             {#if !isOwner}· by {pl.owner_username}{/if}
           </p>
         </div>
-        {#if pl.system_variant === 'discover'}
+        {#if pl.refreshable && pl.system_variant}
           <button
             type="button"
-            disabled={refreshingDiscover}
-            onclick={onRefreshDiscover}
+            disabled={refreshingSystem}
+            onclick={() => onRefreshSystem(pl.system_variant as string)}
             class="rounded-md border border-border px-3 py-1.5 text-sm text-text-muted hover:text-text-primary"
-            aria-label="Refresh Discover"
+            aria-label="Refresh playlist"
           >
-            {refreshingDiscover ? 'Refreshing…' : 'Refresh'}
+            {refreshingSystem ? 'Refreshing…' : 'Refresh'}
           </button>
         {/if}
         {#if isOwner}