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.

docs/src/overview.md

@@ -11,6 +11,7 @@ this book is built from `docs/` and served on `control` over the ZeroTier mesh.
 | `control` | ZeroTier controller, monitoring, docs  | `77.42.68.181`   | `2a01:4f9:c013:e6d0::1`     |
 | `ns1`     | Knot DNS **primary** (master)          | `46.224.170.206` | `2a01:4f8:c014:b5c5::1`     |
 | `ns2`     | Knot DNS **secondary** (slave)         | `157.180.70.82`  | `2a01:4f9:c014:6d87::1`     |
+| `mx1`     | Mail server (**MX** for cnx.email)     | `5.223.65.38`    | `2a01:4ff:2f0:1963::1`      |
 
 ## Access