Fix Dovecot passwd-file format and document mbox mail import

- passwd.j2: append '::::::' so entries have full 8-field passwd format.
  Without this Dovecot's userdb lookup fails ('user is missing userdb
  info') even though passdb auth succeeds, causing login to return 401.
- main.yml: fix passwd file mode from 0640 to 0644 so Dovecot can read it.
- Add runbook-mail-import.md: full procedure for migrating mbox-format mail
  (old Dovecot server with ~/mail/ flat files) to Maildir++ including the
  conversion script that handles fetchmail >From envelope lines.
- runbook-configuration.md: reference new runbook in the mail import section.

ansible/roles/mail/templates/passwd.j2

@@ -1,4 +1,4 @@
 # Managed by Ansible — do not edit manually
 {% for user in dovecot_users | default([]) %}
-{{ user.username }}:{SHA512-CRYPT}{{ user.password | password_hash('sha512', (user.username | hash('md5'))[:16]) }}
+{{ user.username }}:{SHA512-CRYPT}{{ user.password | password_hash('sha512', (user.username | hash('md5'))[:16]) }}::::::
 {% endfor %}