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.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test, vi, beforeEach } from 'vitest';
-import { render, screen, fireEvent } from '@testing-library/svelte';
+import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
 import PlaylistCard from './PlaylistCard.svelte';
 import type { Playlist, PlaylistDetail } from '$lib/api/types';
 
@@ -7,6 +7,14 @@ vi.mock('$lib/auth/store.svelte', () => ({
   user: { value: { id: 'u-self', username: 'me', is_admin: false } }
 }));
 
+vi.mock('@tanstack/svelte-query', async (orig) => {
+  const actual = (await orig()) as Record<string, unknown>;
+  return {
+    ...actual,
+    useQueryClient: () => ({ invalidateQueries: vi.fn().mockResolvedValue(undefined) })
+  };
+});
+
 vi.mock('$lib/api/playlists', () => ({
   getPlaylist: vi.fn().mockResolvedValue({
     id: 'p-1',
@@ -37,13 +45,21 @@ vi.mock('$lib/api/playlists', () => ({
         added_at: '2026-01-01T00:00:00Z'
       }
     ]
-  } as PlaylistDetail)
+  } as PlaylistDetail),
+  refreshDiscover: vi.fn().mockResolvedValue({ playlist_id: 'p-1', track_count: 5 })
 }));
 
 vi.mock('$lib/player/store.svelte', () => ({
   playQueue: vi.fn()
 }));
 
+vi.mock('$lib/api/queries', () => ({
+  qk: {
+    playlist: (id: string) => ['playlist', id],
+    playlists: (kind?: string) => ['playlists', { kind: kind ?? 'user' }]
+  }
+}));
+
 const base: Playlist = {
   id: 'p-1',
   user_id: 'u-self',
@@ -133,4 +149,71 @@ describe('PlaylistCard', () => {
     expect(getPlaylist).toHaveBeenCalledWith('p-1');
     expect(playQueue).toHaveBeenCalled();
   });
+
+  test('shows kebab button only on Discover playlists', () => {
+    const discoverPlaylist: Playlist = {
+      ...base,
+      kind: 'system',
+      system_variant: 'discover',
+      name: 'Discover'
+    };
+    render(PlaylistCard, { props: { playlist: discoverPlaylist } });
+    expect(screen.getByLabelText(/playlist actions/i)).toBeInTheDocument();
+  });
+
+  test('does not show kebab button on for_you playlists', () => {
+    const forYouPlaylist: Playlist = {
+      ...base,
+      kind: 'system',
+      system_variant: 'for_you',
+      name: 'For You'
+    };
+    render(PlaylistCard, { props: { playlist: forYouPlaylist } });
+    expect(screen.queryByLabelText(/playlist actions/i)).not.toBeInTheDocument();
+  });
+
+  test('does not show kebab button on user playlists', () => {
+    render(PlaylistCard, { props: { playlist: base } });
+    expect(screen.queryByLabelText(/playlist actions/i)).not.toBeInTheDocument();
+  });
+
+  test('Refresh Discover item appears in kebab menu when opened', async () => {
+    const discoverPlaylist: Playlist = {
+      ...base,
+      kind: 'system',
+      system_variant: 'discover',
+      name: 'Discover'
+    };
+    render(PlaylistCard, { props: { playlist: discoverPlaylist } });
+    const kebabBtn = screen.getByLabelText(/playlist actions/i);
+    await fireEvent.click(kebabBtn);
+    expect(screen.getByRole('menuitem', { name: /refresh discover/i })).toBeInTheDocument();
+  });
+
+  test('Refresh Discover item is absent from for_you card kebab', () => {
+    const forYouPlaylist: Playlist = {
+      ...base,
+      kind: 'system',
+      system_variant: 'for_you',
+      name: 'For You'
+    };
+    render(PlaylistCard, { props: { playlist: forYouPlaylist } });
+    expect(screen.queryByRole('menuitem', { name: /refresh discover/i })).not.toBeInTheDocument();
+  });
+
+  test('clicking Refresh Discover calls refreshDiscover', async () => {
+    const { refreshDiscover } = await import('$lib/api/playlists');
+    const discoverPlaylist: Playlist = {
+      ...base,
+      kind: 'system',
+      system_variant: 'discover',
+      name: 'Discover'
+    };
+    render(PlaylistCard, { props: { playlist: discoverPlaylist } });
+    const kebabBtn = screen.getByLabelText(/playlist actions/i);
+    await fireEvent.click(kebabBtn);
+    const refreshItem = screen.getByRole('menuitem', { name: /refresh discover/i });
+    await fireEvent.click(refreshItem);
+    await waitFor(() => expect(refreshDiscover).toHaveBeenCalled());
+  });
 });