Split the HTTP plumbing from the business logic in the v1 project-background
download and Unsplash image proxy handlers so /api/v2 can reuse it without
duplicating it:
- LoadProjectBackgroundForDownload (background/handler) loads the bg file +
modtime and fires the Unsplash pingback; GetProjectBackground now calls it.
- WriteProjectBackground (web/files) writes v1's exact background wire shape
(image/jpg, no-cache, stat-modtime Last-Modified, If-Modified-Since 304).
- FetchUnsplashImageByID / FetchUnsplashThumbByID (background/unsplash) return
the open upstream body for the caller to stream; the v1 proxy handlers now
call them. A typed ErrUnsplashImageDoesNotExist maps to 404 on both APIs.
- ErrProjectHasNoBackground (models) gives the no-background case a domain
error; v1 keeps its verbatim 404 message.
v1 responses are unchanged on the wire.
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