feat(nix): add marpa as binary cache for Nix

system/services/harmonia.nix

@@ -0,0 +1,36 @@
+{
+  config,
+  lib,
+  ...
+}:
+with lib; let
+  cfg = config.mySystem.services.harmonia;
+in {
+  options.mySystem.services.harmonia = {
+    enable = mkEnableOption "Harmonia Nix binary cache server";
+    port = mkOption {
+      type = types.port;
+      default = 5000;
+      description = "Port to listen on";
+    };
+    priority = mkOption {
+      type = types.ints.between 0 100;
+      default = 50;
+      description = "Cache priority (lower = higher priority, 0-100)";
+    };
+    signKeyPaths = mkOption {
+      type = types.listOf types.path;
+      description = "Paths to the signing keys to use for signing the cache.";
+    };
+  };
+  config = mkIf cfg.enable {
+    services.harmonia.cache = {
+      enable = true;
+      inherit (cfg) signKeyPaths;
+      settings = {
+        inherit (cfg) priority;
+        bind = "[::]:${toString cfg.port}";
+      };
+    };
+  };
+}