mailmig: Stalwart -> SNM IMAP migration tool (initial)

tests/expected-dovecot-master-user.nix

@@ -0,0 +1,20 @@
+# TEMPORARY: Dovecot master user for the mail migration (mailmig).
+# Lets "<account>*mailmig" log in to any mailbox with the master password.
+# Import this module on the SNM host only while migrating, then remove it.
+#
+# Create the passwd file on the host (root, 0640 root:dovecot2):
+#   printf 'mailmig:{CRYPT}%s\n' "$(mkpasswd -m sha-512)" > /var/lib/dovecot-master-users
+{ ... }:
+{
+  services.dovecot2.settings = {
+    auth_master_user_separator = "*";
+    "passdb mailmig-master" = {
+      driver = "passwd-file";
+      passwd_file_path = "/var/lib/dovecot-master-users";
+      master = true;
+      # After the master password checks out, still require the target
+      # account to exist in SNM's own passdb.
+      result_success = "continue";
+    };
+  };
+}