Artist-first add + mobile API contract

The Add surface has a new default. Artist-first search is now the hero — type an artist name, pick your release, done. Camera, barcode, and upload are still there, tucked behind a toggle rather than sitting inline. This reverses the original camera-first layout.

The search is backed by a local MusicBrainz-derived catalog: 92.6% artist coverage, 63.9% album coverage. Type-ahead is wired to the new API layer, not a live external call.

That API layer — the mobile contract — shipped alongside: OpenAPI spec, delta sync, idempotent writes, boundary validation. It’s the shared foundation artist-first runs on and the iOS companion will build from.