Add site gateway role (modules/router) and gw-cnx-1

Reusable cnx.router.* module for the Topton 1U boxes replacing OPNsense:
PPPoE WAN (optionally VLAN-tagged, AIS: 10, secret credentials incl.
username), VLAN-filtering bridge, nftables NAT/firewall with MSS clamp,
Kea DHCP with per-VLAN lease time, Blocky DNS, DHCPv6-PD, CrowdSec with
the ZeroTier mesh whitelisted, optional Omada controller, ZFS disk.

Fleet baseline rides along: admins sops group is now derived for every
machine in clan.nix (secrets encrypt to it from the first vars generate)
and time sync is chrony everywhere instead of systemd-timesyncd.

modules/router/crowdsec.nix

@@ -0,0 +1,52 @@
+# CrowdSec security engine + nftables bouncer: parses sshd auth attempts from
+# the journal and bans offending source IPs at the firewall. Log-based (no
+# inline DPI) so it costs the N300 next to nothing.
+{
+  config,
+  lib,
+  ...
+}:
+let
+  cfg = config.cnx.router;
+  mesh = import ../mesh-hosts.nix { inherit config lib; };
+in
+{
+  config = lib.mkIf cfg.enable {
+    services.crowdsec = {
+      enable = true;
+      autoUpdateService = true;
+      hub.collections = [
+        "crowdsecurity/linux"
+        "crowdsecurity/sshd"
+      ];
+      localConfig = {
+        acquisitions = [
+          {
+            source = "journalctl";
+            journalctl_filter = [ "_SYSTEMD_UNIT=sshd.service" ];
+            labels.type = "syslog";
+          }
+        ];
+        # Never ban the ZeroTier mesh — it is the only admin path to these
+        # boxes (no public SSH), so a false positive would lock us out.
+        # Parser-stage whitelist: mesh events are dropped before any scenario.
+        parsers.s02Enrich = [
+          {
+            name = "cnx/mesh-whitelist";
+            description = "Whitelist the ZeroTier management mesh";
+            whitelist = {
+              reason = "ZeroTier mesh is the admin path";
+              cidr = [ mesh.subnet ];
+            };
+          }
+        ];
+      };
+    };
+
+    services.crowdsec-firewall-bouncer = {
+      enable = true;
+      registerBouncer.enable = true;
+      settings.mode = "nftables";
+    };
+  };
+}