Onboard mx1 mail host and factor out per-host public IPs

- Register mx1 in the inventory and as a direct-SSH `internet` host; give it
  a static public IPv6 (2a01:4ff:2f0:1963::1).
- Point the cnx.email MX (plus SPF/DMARC) at mx1 and add its A record.
- Bring mx1 into monitoring: import exporters, add it to the mesh map and the
  node scrape job so its host metrics and journald reach control.
- Add a clan-mx1 Hetzner firewall: inbound SMTP + ZeroTier + ICMP, no public
  SSH (admin rides the mesh like the other hosts). 587/465/993 held for now.
- Extract per-host public IPv4/IPv6 into modules/hosts.nix, consumed by
  clan.nix's internet hosts and each machine's cnx.staticIPv6, so each address
  is declared once instead of being duplicated across configs.
- docs: add mx1 to the machines table.

machines/control/configuration.nix

@@ -1,3 +1,7 @@
+{ config, ... }:
+let
+  hosts = import ../../modules/hosts.nix;
+in
 {
   imports = [
     ../../modules/hetzner-firewall.nix
@@ -11,10 +15,10 @@
 
   clan.core.sops.defaultGroups = [ "admins" ];
 
-  # Public IPv6; SLAAC doesn't bring it up here.
+  # Public IPv6 (from modules/hosts.nix); SLAAC doesn't bring it up here.
   cnx.staticIPv6 = {
     enable = true;
-    address = "2a01:4f9:c013:e6d0::1";
+    address = hosts.${config.networking.hostName}.ipv6;
   };
 
   time.timeZone = "Etc/GMT-3"; # UTC+3 (fixed offset, no DST)