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>
