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/api/playlists.refresh-discover.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { refreshDiscover } from './playlists';
+
+vi.mock('./client', () => ({
+  apiFetch: vi.fn()
+}));
+
+import { apiFetch } from './client';
+
+describe('refreshDiscover', () => {
+  beforeEach(() => vi.clearAllMocks());
+
+  it('POSTs the correct path and returns the response', async () => {
+    const sample = { playlist_id: 'abc-123', track_count: 100 };
+    (apiFetch as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce(sample);
+    const got = await refreshDiscover();
+    expect(apiFetch).toHaveBeenCalledWith(
+      '/api/playlists/system/discover/refresh',
+      expect.objectContaining({ method: 'POST' })
+    );
+    expect(got).toEqual(sample);
+  });
+
+  it('handles empty-library response', async () => {
+    const sample = { playlist_id: null, track_count: 0 };
+    (apiFetch as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce(sample);
+    const got = await refreshDiscover();
+    expect(got.playlist_id).toBe(null);
+    expect(got.track_count).toBe(0);
+  });
+});