Add internal reverse proxy for gateways (Caddy, wildcard via DNS-01)

cnx.router.proxy serves <service>.<site><n>.cnx.network with a real Let's
Encrypt wildcard obtained via a gateway-scoped TSIG key against ns1; Blocky
resolves the names to the router's LAN address, so they exist only
internally. First user: Omada UI on gw-cnx-1 (omada.cnx1.cnx.network).

modules/dns/acme-gw-secret.nix

@@ -0,0 +1,17 @@
+# Shared TSIG secret for a gateway's dedicated ACME key (function: machine
+# name -> module). The acme_gw_<x> key lets that gateway — and only it — write
+# _acme-challenge.<label> TXT records on ns1 to obtain its internal wildcard
+# cert via DNS-01. ns1 scopes it with a matching acl on the cnx.network zone.
+# Import on BOTH ns1 and the gateway machine, applied with the machine name:
+#   (import ../../modules/dns/acme-gw-secret.nix "gw-cnx-1")
+machine:
+{ pkgs, ... }:
+{
+  clan.core.vars.generators."dns-acme-${machine}-secret" = {
+    share = true;
+    files."secret".secret = true;
+    runtimeInputs = [ pkgs.openssl ];
+    # 32 random bytes, base64 — a valid hmac-sha256 TSIG secret.
+    script = ''openssl rand -base64 32 | tr -d '\n' > "$out"/secret'';
+  };
+}