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)

outputs/x86_64-linux/tests/youko-nix-cache-proxy/expected.nix

@@ -0,0 +1,11 @@
+{
+  nginxEnabled = true;
+  caddyVhostExists = true;
+  ustcRemainsFirst = true;
+  nixosProxyConfigured = true;
+  numtideProxyConfigured = true;
+  numtideDirectFallback = true;
+  cacheLivesOnDataVolume = true;
+  nixosUpstream = "https://cache.nixos.org/";
+  numtideUpstream = "https://cache.numtide.com/";
+}

outputs/x86_64-linux/tests/youko-nix-cache-proxy/expr.nix

@@ -0,0 +1,21 @@
+{
+  lib,
+  outputs,
+  ...
+}:
+let
+  cfg = outputs.nixosConfigurations.youko.config;
+  nginx = cfg.services.nginx;
+  substituters = cfg.nix.settings.substituters;
+in
+{
+  nginxEnabled = nginx.enable;
+  caddyVhostExists = cfg.services.caddy.virtualHosts ? "nix-cache.writefor.fun";
+  ustcRemainsFirst = builtins.head substituters == "https://mirrors.ustc.edu.cn/nix-channels/store";
+  nixosProxyConfigured = builtins.elem "https://nix-cache.writefor.fun/cache.nixos.org" substituters;
+  numtideProxyConfigured = builtins.elem "https://nix-cache.writefor.fun/cache.numtide.com" substituters;
+  numtideDirectFallback = builtins.elem "https://cache.numtide.com" substituters;
+  cacheLivesOnDataVolume = lib.hasInfix "/data/apps/nix-cache-proxy" nginx.appendHttpConfig;
+  nixosUpstream = nginx.virtualHosts."nix-cache-proxy".locations."/cache.nixos.org/".proxyPass;
+  numtideUpstream = nginx.virtualHosts."nix-cache-proxy".locations."/cache.numtide.com/".proxyPass;
+}