Add configurable SMTP TLS/SSL options (#5204)

## Summary
- add optional STARTTLS and SSL-related fields to mail settings with
defaults matching prior behavior
- apply the new mail properties in the SMTP JavaMail configuration,
including trust and hostname verification overrides
- update mail settings template and tests to cover default behavior and
explicit TLS/SSL overrides
- clarify STARTTLS naming and sslTrust usage with examples in property
comments and the settings template
- default sslTrust to a wildcard when unset so TLS connections accept
any host by default unless tightened

## Testing
- ./gradlew :proprietary:test --tests
stirling.software.proprietary.security.service.MailConfigTest --console
plain

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[Codex
Task](https://chatgpt.com/codex/tasks/task_b_693864b2a6648328ae75c7e88a726a65)

app/core/src/main/resources/settings.yml.template

@@ -104,6 +104,11 @@ mail:
   username: '' # SMTP server username
   password: '' # SMTP server password
   from: '' # sender email address
+  startTlsEnable: true # enable STARTTLS (explicit TLS upgrade after connecting) when supported by the SMTP server
+  startTlsRequired: false # require STARTTLS; connection fails if the upgrade command is not supported
+  sslEnable: false # enable SSL/TLS wrapper for implicit TLS (typically used with port 465)
+  sslTrust: '' # optional trusted host override, e.g. "smtp.example.com" or "*"; defaults to "*" (trust all) when empty
+  sslCheckServerIdentity: false # enable hostname verification when using SSL/TLS
 
 legal:
   termsAndConditions: https://www.stirling.com/legal/terms-of-service # URL to the terms and conditions of your application (e.g. https://example.com/terms). Empty string to disable or filename to load from local file in static folder