feat(web/playlists): Discover refresh button (detail + home kebab)

Operator can refresh their Discover playlist without waiting for
the next daily cron tick. Two surfaces, both gated on
system_variant === 'discover':

- Detail page header: "Refresh" button next to the cover art / meta.
- Home Playlists row tile kebab: "Refresh Discover" menu item.

Both call POST /api/playlists/system/discover/refresh, show a
confirmation toast, and invalidate the playlist + playlists queries
so the new content lands without a manual page reload.

Extends Playlist.system_variant union with 'discover'. Adds
refreshDiscover() typed client. Tests cover client behaviour
(success + empty-library) plus conditional rendering on both
surfaces.

web/src/routes/playlists/[id]/+page.svelte

@@ -11,7 +11,8 @@
     updatePlaylist,
     deletePlaylist,
     removePlaylistTrack,
-    reorderPlaylist
+    reorderPlaylist,
+    refreshDiscover
   } from '$lib/api/playlists';
   import { qk } from '$lib/api/queries';
   import { user } from '$lib/auth/store.svelte';
@@ -135,12 +136,49 @@
       alert(copyForCode(code));
     }
   }
+
+  // --- Toast (detail page) ---
+  let toast = $state<string | null>(null);
+  let toastTimer: ReturnType<typeof setTimeout> | null = null;
+
+  function showToast(msg: string) {
+    if (toastTimer) clearTimeout(toastTimer);
+    toast = msg;
+    toastTimer = setTimeout(() => { toast = null; }, 5000);
+  }
+
+  // --- Discover refresh ---
+  let refreshingDiscover = $state(false);
+
+  async function onRefreshDiscover() {
+    refreshingDiscover = true;
+    try {
+      await refreshDiscover();
+      showToast('Discover refreshed.');
+      await queryClient.invalidateQueries({ queryKey: qk.playlist(id) });
+      await queryClient.invalidateQueries({ queryKey: qk.playlists() });
+    } catch (e: unknown) {
+      showToast(`Refresh failed: ${(e as { code?: string })?.code ?? 'unknown'}`);
+    } finally {
+      refreshingDiscover = false;
+    }
+  }
 </script>
 
 <svelte:head>
   <title>{pageTitle(playlistQuery?.data?.name ? `Playlist · ${playlistQuery.data.name}` : 'Playlist')}</title>
 </svelte:head>
 
+{#if toast}
+  <div
+    role="status"
+    aria-live="polite"
+    class="fixed bottom-4 left-1/2 z-50 -translate-x-1/2 rounded-md border border-border bg-surface px-4 py-2 text-sm text-text-primary shadow-md"
+  >
+    {toast}
+  </div>
+{/if}
+
 <div class="mx-auto max-w-4xl px-4 py-6">
   {#if playlistQuery?.isPending}
     <p class="text-text-muted">Loading…</p>
@@ -166,6 +204,17 @@
             {#if !isOwner}· by {pl.owner_username}{/if}
           </p>
         </div>
+        {#if pl.system_variant === 'discover'}
+          <button
+            type="button"
+            disabled={refreshingDiscover}
+            onclick={onRefreshDiscover}
+            class="rounded-md border border-border px-3 py-1.5 text-sm text-text-muted hover:text-text-primary"
+            aria-label="Refresh Discover"
+          >
+            {refreshingDiscover ? 'Refreshing…' : 'Refresh'}
+          </button>
+        {/if}
         {#if isOwner}
           <button
             type="button"