Download a public asset

Serves the raw bytes of a stored asset from its stable tenant-scoped, content-addressed public URL. The response Content-Type is whatever the asset was stored as. Because the asset id is the SHA-256 content hash, the body can never change for a given URL, so the response is served with Cache-Control: public, max-age=31536000, immutable. Returns 404 for an unknown tenant, namespace, or asset.