feat: implement comprehensive postfix gateway with eFa5 configuration

- add voxpupuli-postfix module to Puppetfile
- create profiles::postfix::gateway class with config based on efa5
- add master.cf entries for postscreen, smtpd, dnsblog, and tlsproxy services
- create postfix hash files: aliases, access controls, canonical maps
- configure TLS with system PKI certificates and strong cipher suites
- add transport and virtual alias mappings for mail routing

Puppetfile

@@ -43,6 +43,8 @@ mod 'puppet-letsencrypt', '11.1.0'
 mod 'puppet-rundeck', '9.2.0'
 mod 'puppet-redis', '11.1.0'
 mod 'puppet-nodejs', '11.0.0'
+mod 'puppet-postfix', '5.1.0'
+mod 'puppet-alternatives', '6.0.0'
 
 # other
 mod 'saz-sudo', '9.0.2'