Handle failed responses in fetchArtistFromSpotify

fetchArtistFromSpotify (backend/src/helpers/fetchArtistFromSpotify.ts) never checks response.ok. If Spotify returns a 404 or a 429, the error JSON gets cast to SpotifyArtist and flows into the image comparison logic in the artist image update job, producing garbage instead of a visible failure.

• Check response.ok and return null (or throw) on failure so the caller's existing null handling kicks in
• Drop the redundant url parameter, every caller builds it from the id they already pass in