feat(youko): add pull-through cache for two Nix upstreams (#499)

Run nginx on youko as a loopback pull-through HTTP cache for
cache.nixos.org and cache.numtide.com, exposed by Caddy at
https://nix-cache.writefor.fun.

Assisted-by: DeepSeek Harness (deepseek-flash)

docs/nixos-modernization-options.md

@@ -199,8 +199,7 @@ defaults become less transparent than a file you can read in the diff; your deli
 
 ## A7. attic self-hosted binary cache
 
-**Current:** substituters are the two Chinese mirrors plus numtide's cache; a cachix endpoint exists
-in the config but is unused.
+**Current:** substituters are the two Chinese mirrors plus numtide's cache.
 
 **Proposed:** `services.networking.atticd` on youko.