feat(substituters): Implement client-side binary cache

modules/common/substituters.nix

@@ -0,0 +1,28 @@
+{ config, lib, pkgs, inputs, ... }:
+
+let
+  cfg = config.homelab.common.substituters;
+in {
+  options.homelab.common.substituters = {
+    enable = lib.mkEnableOption "Binary cache substituters";
+    domain = lib.mkOption {
+      type = lib.types.str;
+      default = inputs.self.nixosConfigurations.BinaryCache.config.homelab.services.attic.domain;
+      description = "The domain name of the binary cache.";
+    };
+    publicKey = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      description = "The public key of the Attic cache (e.g., 'homelab:...')";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    nix.settings = {
+      substituters = [
+        "https://${cfg.domain}"
+      ];
+      trusted-public-keys = lib.optional (cfg.publicKey != null) cfg.publicKey;
+    };
+  };
+}