Add parsedmarc DMARC report analyzer on control

Deliver cnx.email DMARC aggregate/forensic reports to a dedicated dmarc@cnx.email
mailbox on mx1 and analyze them with parsedmarc on control, storing parsed
reports in a local loopback Elasticsearch and visualizing via the auto-provisioned
Grafana dashboard. parsedmarc fetches the mailbox over IMAPS across the mesh
(mx1.cnx.email pinned to its mesh address so TLS still validates), using a shared
mail-dmarc-cred clan var so mx1's mailserver and control see the same password.

modules/mail-dmarc-cred.nix

@@ -0,0 +1,26 @@
+# Shared credential for the dmarc@cnx.email mailbox.
+#
+# DMARC aggregate/forensic reports are delivered to dmarc@cnx.email on mx1;
+# parsedmarc on control fetches them over IMAPS across the mesh and needs the
+# *plaintext* passphrase, while mx1's mailserver only needs the sha-512 hash.
+# clan vars secrets are per-machine, so this generator is shared (share = true)
+# to make the same value available on both hosts. Files are root-owned: SNM reads
+# the hash as root, and parsedmarc's ExecStartPre reads the passphrase as root.
+# Imported by mx1 (via mail.nix) and control (via monitoring/parsedmarc.nix).
+{ pkgs, ... }:
+{
+  clan.core.vars.generators.mail-dmarc-cred = {
+    share = true;
+    files."passphrase".secret = true; # read by parsedmarc on control
+    files."hash".secret = true; # consumed by the mailserver on mx1
+    runtimeInputs = [
+      pkgs.xkcdpass
+      pkgs.mkpasswd
+    ];
+    script = ''
+      pass="$(xkcdpass --numwords=4 --delimiter=- --case=lower)-$((RANDOM % 90 + 10))"
+      printf '%s' "$pass" > "$out"/passphrase
+      printf '%s' "$pass" | mkpasswd -s -m sha-512 > "$out"/hash
+    '';
+  };
+}