Brand assets served at /api/files/v1/static had no cache headers.
Introduce brand.Assets to own the embedded filesystem, content-hash
ETags, and HTTP serving. Responses now carry Cache-Control and ETag
so clients can cache and revalidate; stable email URLs stay
revalidatable (max-age=3600, no immutable).
Replace hardcoded Default*Path constants with StaticPathPrefix,
logical filename constants, and StaticPath(). NewAssets validates
required assets at startup so a rename fails fast instead of 404ing
in sent emails. The files handler keeps routing and 404 rendering;
ServeAssets sets cache headers and serves the file.
Signed-off-by: Ludovic Vielle <ludovic@probo.com>
Add GET /public/{fileID} (unauthenticated, public files only) and
GET /{fileID} (session+API key+OAuth2, all files, core:file:get IAM
check). IAM and not-found errors both return 404 to prevent leaking
whether a file exists.
Signed-off-by: Ludovic Vielle <ludovic@probo.com>
