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/lib/components/PlaylistCard.svelte

@@ -1,15 +1,37 @@
 <script lang="ts">
-  import { Play } from 'lucide-svelte';
+  import { Play, MoreVertical, RefreshCcw } from 'lucide-svelte';
+  import { useQueryClient } from '@tanstack/svelte-query';
   import type { Playlist, PlaylistTrack, TrackRef } from '$lib/api/types';
   import { user } from '$lib/auth/store.svelte';
-  import { getPlaylist } from '$lib/api/playlists';
+  import { getPlaylist, refreshDiscover } from '$lib/api/playlists';
+  import { qk } from '$lib/api/queries';
   import { playQueue } from '$lib/player/store.svelte';
 
   let { playlist }: { playlist: Playlist } = $props();
 
   const isOwn = $derived(user.value?.id === playlist.user_id);
+  const isDiscover = $derived(playlist.system_variant === 'discover');
+
+  const queryClient = useQueryClient();
 
   let starting = $state(false);
+  let menuOpen = $state(false);
+
+  // --- Toast ---
+  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);
+  }
+
+  function toggleMenu(e: MouseEvent) {
+    e.preventDefault();
+    e.stopPropagation();
+    menuOpen = !menuOpen;
+  }
 
   // Map PlaylistTrack rows to TrackRef shape and drop tracks whose upstream
   // file has been removed (track_id is null). Mirrors PlaylistTrackRow's
@@ -47,9 +69,67 @@
       starting = false;
     }
   }
+
+  async function onRefreshDiscover(e: MouseEvent) {
+    e.preventDefault();
+    e.stopPropagation();
+    menuOpen = false;
+    try {
+      await refreshDiscover();
+      showToast('Discover refreshed.');
+      await queryClient.invalidateQueries({ queryKey: qk.playlist(playlist.id) });
+      await queryClient.invalidateQueries({ queryKey: qk.playlists() });
+    } catch (err: unknown) {
+      showToast(`Refresh failed: ${(err as { code?: string })?.code ?? 'unknown'}`);
+    }
+  }
 </script>
 
+<svelte:window onclick={() => { menuOpen = false; }} />
+
+{#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="card relative">
+  {#if isDiscover}
+    <div class="kebab-wrap absolute right-1 top-1 z-10">
+      <button
+        type="button"
+        aria-label="Playlist actions"
+        aria-haspopup="menu"
+        aria-expanded={menuOpen}
+        onclick={toggleMenu}
+        class="rounded p-1 text-text-muted hover:bg-surface-hover hover:text-text-primary"
+      >
+        <MoreVertical size={16} strokeWidth={1} />
+      </button>
+      {#if menuOpen}
+        <div
+          role="menu"
+          tabindex="-1"
+          class="absolute right-0 top-full z-20 mt-1 w-44 rounded-md border border-border bg-surface p-1 shadow-lg"
+          onclick={(e) => e.stopPropagation()}
+        >
+          <button
+            type="button"
+            role="menuitem"
+            onclick={onRefreshDiscover}
+            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} />
+            Refresh Discover
+          </button>
+        </div>
+      {/if}
+    </div>
+  {/if}
   <a
     href={`/playlists/${playlist.id}`}
     class="group flex w-full flex-col items-start gap-2 rounded-md p-2 text-left hover:bg-surface-hover"