Add ACME configuration for Let's Encrypt certificates

- Accept Let's Encrypt terms of service
- Configure email for certificate notifications (dlei@duck.com)
- Nginx virtual hosts already configured with enableACME and forceSSL

Ready for deployment to VPS.

configuration.nix

@@ -49,6 +49,12 @@
     allowedTCPPorts = [ 22 80 443 ];  # SSH, HTTP, HTTPS
   };
 
+  # ACME for Let's Encrypt certificates
+  security.acme = {
+    acceptTerms = true;
+    defaults.email = "dlei@duck.com";
+  };
+
   # This value determines the NixOS release compatibility
   system.stateVersion = "24.05";
 }