nix-cache: use hostname-based domain for Caddy proxy

nix-cache01 serves nix-cache.home.2rjus.net (canonical)
nix-cache02 serves nix-cache02.home.2rjus.net (for testing)

This allows testing nix-cache02 independently before DNS cutover.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/nix-cache/proxy.nix

@@ -1,4 +1,14 @@
-{ pkgs, ... }:
+{ pkgs, config, ... }:
+let
+  # nix-cache01 serves the canonical nix-cache.home.2rjus.net
+  # nix-cache02 serves nix-cache02.home.2rjus.net (for testing before DNS cutover)
+  hostname = config.networking.hostName;
+  domain =
+    if hostname == "nix-cache01" then
+      "nix-cache.home.2rjus.net"
+    else
+      "${hostname}.home.2rjus.net";
+in
 {
   services.caddy = {
     enable = true;
@@ -10,7 +20,7 @@
       }
 
 
-      nix-cache.home.2rjus.net {
+      ${domain} {
         log {
           output file /var/log/caddy/nix-cache.log {
             mode 644