feat: implement dovecot backend server with postfix virtual mailbox integration

- create profiles::dovecot::backend class for IMAPS server configuration
- add virtual mailbox support to profiles::postfix::gateway with enable_dovecot parameter
- restructure common hieradata elements into mail.yaml
- add virtual mailbox and alias map templates with ERB generation
- add comprehensive type validation using Stdlib::Email, Stdlib::Fqdn, Stdlib::IP types
- configure vmail user (UID/GID 5000) with shared storage on /shared/apps/maildata
- update roles::infra::mail::backend to include both dovecot and postfix profiles

site/roles/manifests/infra/mail/backend.pp

@@ -6,5 +6,7 @@ class roles::infra::mail::backend {
   }else{
     include profiles::defaults
     include profiles::base
+    include profiles::dovecot::server
+    include profiles::postfix::gateway
   }
 }