Return 404 instead of 500 for unknown album and artist IDs

A few lookups don't handle the not-found case and crash instead of returning a proper 404:

• AlbumService.getAlbumByID (backend/src/api/services/albumService.ts) casts the DB result to ReviewedAlbum without checking it exists, then dereferences it. Any unknown album ID in the URL gives a 500 and a stack trace in the logs. deleteAlbum and updateAlbumReview already do this check properly, so match those.
• ArtistService.updateArtistHeaders and updateArtistImages build [await getArtistBySpotifyID(id)] when given a single spotifyID. If the artist doesn't exist that's [undefined], and the loop crashes partway through the job. Check the lookup result and 404 before starting.