Port the remaining read-only background blob endpoints to /api/v2:
- GET /projects/{project}/background streams the stored background (project
CanRead, in-handler), modeled as an image/jpeg binary response. Honors
If-Modified-Since (304) and serves through the shared WriteProjectBackground.
- GET /backgrounds/unsplash/images/{image} and .../thumb proxy the upstream
Unsplash image through the SSRF-safe client, gated on the unsplash provider
like the sibling unsplash routes, modeled as image/jpeg binary responses.
All three reuse the v1 business logic extracted in the previous commit.
5ccbd0d74e