feat(postfix): implement virtual mailbox configuration for Dovecot LMTP

Updates the Postfix configuration to correctly handle virtual domains when Dovecot LMTP is enabled, moving away from local system delivery settings.

- Removes `postfix_mail_domain` from `postfix_mydestination` to prevent conflicts with virtual domain handling.
- Updates `main.cf` to set `virtual_transport`, `virtual_mailbox_domains`, and `virtual_mailbox_maps` instead of `mailbox_transport`.
- Adds a new template `virtual_mailbox_maps.j2` to authorize specific users defined in `dovecot_users`.
- Adds tasks to generate the virtual mailbox map file and run `postmap` upon changes.

templates/main.cf.j2

@@ -51,9 +51,6 @@ alias_database = hash:/etc/aliases
 
 # Dovecot Integration
 {% if dovecot_enabled | default(false) %}
-# Use Maildir format
-home_mailbox = Maildir/
-
 {% if dovecot_postfix_sasl_enable | default(false) %}
 # SASL Authentication via Dovecot
 smtpd_sasl_type = dovecot
@@ -62,7 +59,9 @@ smtpd_sasl_auth_enable = yes
 {% endif %}
 
 {% if dovecot_postfix_lmtp_enable | default(false) %}
-# Delivery via LMTP
-mailbox_transport = lmtp:unix:private/dovecot-lmtp
+# Virtual mailbox configuration for Dovecot users
+virtual_mailbox_domains = {{ postfix_mail_domain }}
+virtual_mailbox_maps = hash:/etc/postfix/virtual_mailbox_maps
+virtual_transport = lmtp:unix:private/dovecot-lmtp
 {% endif %}
 {% endif %}