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

.gitignore

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