## 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.
- Splits into two env-configurable settings, since which hosts count as trusted providers -- and this platform's own render host -- are both deployment-specific, not just per-environment:
  - `CHROMA_KEY_BGCOLOR_HOSTS` (overlays.uno et al.): appends `?bgcolor=`.
  - `CHROMA_KEY_MODE_HOSTS` (frontend-overlays' own render pages, now defaulting to `plevion.com`): appends `?mode=chromakey` instead -- a completely different convention that was being conflated with the bgcolor one. `render.overlays.plevion.com` is kept as a fallback in case a deployment does use that subdomain.

Pairs with frontend-overlays' own fix wiring `?mode=chromakey` into the scoreboard/match-info render pages, which never read it at all before this.