Port to /api/v2:
- DELETE /projects/{project}/background (remove background, returns the updated project)
- GET /backgrounds/unsplash/search (q, page; gated on the unsplash provider)
- PUT /projects/{project}/backgrounds/unsplash (set, gated on the unsplash provider)
Custom routes load the project and enforce CanUpdate explicitly. Backgrounds
are gated on the static backgrounds config via a registrar early-return.
Tag background.Image fields with doc: for the v2 schema, and add a scoped
contextcheck exclusion since the unsplash provider's shared interface bottoms
out in context.Background().
kolaente