Add CDN system that serves release assets via a dedicated hostname
(e.g., cdn.mokoconsulting.tech) with per-asset public/private toggles,
IP/referrer allowlists, and aggressive caching headers.
- Host-based routing intercepts CDN domain before auth middleware
- Per-attachment cdn_public flag controls CDN visibility
- Releases in an update stream are excluded from CDN (update server takes precedence)
- CORS, ETag, Cache-Control headers for downstream CDN compatibility
- IP/CIDR and referrer domain allowlists for abuse prevention