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.

modules/mesh-hosts.nix

@@ -14,7 +14,7 @@ let
     machine: file:
     builtins.readFile "${dir}/vars/per-machine/${machine}/zerotier/${file}/value";
 
-  hosts = lib.genAttrs [ "control" "ns1" "ns2" ] (m: readVar m "zerotier-ip");
+  hosts = lib.genAttrs [ "control" "ns1" "ns2" "mx1" ] (m: readVar m "zerotier-ip");
 
   # RFC 4193 prefix of this ZeroTier network: fd + the 8-byte network id + the
   # 0x9993 marker. The network id is a public var on the controller (control).