## Summary
- `with_chroma_key_background`'s host allowlist was a hardcoded module constant naming `render.overlays.plevion.com` -- which doesn't exist; confirmed live the actual production render host is `plevion.com` (Kong's path-based `/overlays` routing, not a separate subdomain), so the function always returned overlay URLs completely unchanged for this platform's own render pages.
- Makes the host set env-configurable (`CHROMA_KEY_PROVIDER_HOSTS`), since which hosts count as trusted providers -- and this platform's own render host -- are both deployment-specific, not just per-environment. `render.overlays.plevion.com` is kept as a fallback in case a deployment does use that subdomain.
- One query-string convention (`?bgcolor=`) resolved the same way regardless of provider -- overlays.uno and frontend-overlays' own render pages both use it now, rather than branching sink's logic per host with two different param conventions.

Pairs with frontend-overlays' own follow-up: scoreboard's render page reads and validates `?bgcolor=` against its own known set of background options (`SCOREBOARD_BACKGROUND_OPTIONS`).