feat(web): extend ArtistRef/AlbumRef + add HomePayload + new query keys

Adds sort_name/cover_url to ArtistRef, sort_title to AlbumRef (matching
backend Task 5 wire shape), HomePayload type, and qk.home/albumsAlpha/
artistTracks query keys. Updates existing test fixtures to satisfy the
new required fields.

Verified clean via 'npm run check' (0 errors, only pre-existing warnings).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/api/queries.ts

@@ -34,6 +34,9 @@ export const qk = {
     ['adminQuarantineActions', { limit: limit ?? 50 }] as const,
   suggestions:            (limit?: number) =>
     ['suggestions', { limit: limit ?? 12 }] as const,
+  home:         ()                 => ['home'] as const,
+  albumsAlpha:  ()                 => ['albumsAlpha'] as const,
+  artistTracks: (artistId: string) => ['artistTracks', artistId] as const,
 };
 
 export function createArtistsQuery(sort: ArtistSort) {