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/profiles/templates/postfix/gateway/virtual_alias_maps.erb

@@ -0,0 +1,9 @@
+# FILE MANAGED BY PUPPET, CHANGES WILL BE REPLACED
+#
+# Defines virtual alias mappings
+# Maps email addresses or patterns to target addresses
+# Format: alias@foo.net    real@corp.foo.net
+
+<% @virtual_alias_maps.each do |source, target| -%>
+<%= source %>       <%= target %>
+<% end -%>

site/profiles/templates/postfix/gateway/virtual_mailbox_maps.erb

@@ -0,0 +1,9 @@
+# FILE MANAGED BY PUPPET, CHANGES WILL BE REPLACED
+#
+# Defines virtual mailbox mappings for dovecot delivery
+# Maps email addresses to maildir paths
+# Format: user@domain    maildir/path/
+
+<% @virtual_mailbox_maps.each do |email, path| -%>
+<%= email %>       <%= path %>
+<% end -%>