Split the generic file-download writer (ServeContent for seekable readers,
manual 304 + io.Copy otherwise) out of WriteAttachmentDownload so other blob
endpoints can reuse it. The attachment writer keeps its preview branch and
cache override and delegates the rest.
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.
kolaente