Add declarative SNM mail stack on mx1 with DNS-01, DANE, MTA-STS

mx1 runs Simple NixOS Mailserver (Postfix/Dovecot/Rspamd/OpenDKIM) for
cnx.email. The TLS cert is obtained via ACME DNS-01 using a dedicated,
scoped TSIG key (acme_mx1) that ns1 authorizes for only
_acme-challenge.mx1 and _acme-challenge.mta-sts on the cnx.email zone, so
the credential can write nothing else. Mailbox passwords are auto-minted
by a clan vars generator (four-word passphrase + number).

DANE TLSA (3 1 1) is published for _25._tcp.mx1; --reuse-key keeps the
key digest stable across renewals. MTA-STS is enforced via a Caddy vhost
serving the policy on :443 from the same cert (mta-sts SAN). Firewall
opens 25/587/465/143/993/443; 80 stays closed.

flake.lock

@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "blobs": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1604995301,
+        "narHash": "sha256-wcLzgLec6SGJA8fx1OEN1yV/Py5b+U5iyYpksUY/yLw=",
+        "owner": "simple-nixos-mailserver",
+        "repo": "blobs",
+        "rev": "2cccdf1ca48316f2cfd1c9a0017e8de5a7156265",
+        "type": "gitlab"
+      },
+      "original": {
+        "owner": "simple-nixos-mailserver",
+        "repo": "blobs",
+        "type": "gitlab"
+      }
+    },
     "clan-core": {
       "inputs": {
         "data-mesher": "data-mesher",
@@ -73,6 +89,22 @@
         "type": "github"
       }
     },
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1767039857,
+        "narHash": "sha256-vNpUSpF5Nuw8xvDLj2KCwwksIbjua2LZCqhV1LNRDns=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "5edf11c44bc78a0d334f6334cdaf7d60d732daab",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
     "flake-parts": {
       "inputs": {
         "nixpkgs-lib": [
@@ -94,6 +126,54 @@
         "type": "github"
       }
     },
+    "git-hooks": {
+      "inputs": {
+        "flake-compat": [
+          "nixos-mailserver",
+          "flake-compat"
+        ],
+        "gitignore": "gitignore",
+        "nixpkgs": [
+          "nixos-mailserver",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1772893680,
+        "narHash": "sha256-JDqZMgxUTCq85ObSaFw0HhE+lvdOre1lx9iI6vYyOEs=",
+        "owner": "cachix",
+        "repo": "git-hooks.nix",
+        "rev": "8baab586afc9c9b57645a734c820e4ac0a604af9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "cachix",
+        "repo": "git-hooks.nix",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "nixos-mailserver",
+          "git-hooks",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1709087332,
+        "narHash": "sha256-HG2cCnktfHsKV0s4XW83gU3F57gaTljL9KNSuG6bnQs=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
     "nix-darwin": {
       "inputs": {
         "nixpkgs": [
@@ -144,6 +224,30 @@
         "type": "github"
       }
     },
+    "nixos-mailserver": {
+      "inputs": {
+        "blobs": "blobs",
+        "flake-compat": "flake-compat",
+        "git-hooks": "git-hooks",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1773912645,
+        "narHash": "sha256-QHzRqq6gh+t3F/QU9DkP7X63dDDcuIQmaDz12p7ANTg=",
+        "owner": "simple-nixos-mailserver",
+        "repo": "nixos-mailserver",
+        "rev": "25e6dbb8fca3b6e779c5a46fd03bd760b2165bb5",
+        "type": "gitlab"
+      },
+      "original": {
+        "owner": "simple-nixos-mailserver",
+        "ref": "nixos-25.11",
+        "repo": "nixos-mailserver",
+        "type": "gitlab"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1778003029,
@@ -163,6 +267,7 @@
     "root": {
       "inputs": {
         "clan-core": "clan-core",
+        "nixos-mailserver": "nixos-mailserver",
         "nixpkgs": [
           "clan-core",
           "nixpkgs"