Expose SMTP TLS controls for local relays

[realicon23]

Fixes #1186.

Why

Self-hosted Docker deployments can relay mail through an internal SMTP server that advertises STARTTLS with a private or self-signed certificate. Symfony Mailer enables opportunistic STARTTLS by default, so leaving MAIL_ENCRYPTION empty does not necessarily stop the transport from upgrading to STARTTLS.

Laravel passes the SMTP mailer config through to Symfony's SMTP DSN options, and Symfony already supports auto_tls and verify_peer. This keeps the secure defaults in place while giving trusted internal relays an explicit opt-out.

What changed

• Adds MAIL_AUTO_TLS and MAIL_VERIFY_PEER to the SMTP mailer config.
• Passes those variables through the all-in-one Docker compose environment.
• Documents the defaults in the backend and all-in-one env examples.
• Adds a focused unit test proving the options reach Symfony's EsmtpTransport and socket stream.

Verification

/opt/homebrew/opt/php@8.4/bin/php -l backend/config/mail.php
/opt/homebrew/opt/php@8.4/bin/php -l backend/tests/Unit/Configuration/MailConfigurationTest.php
git diff --check
/opt/homebrew/opt/php@8.4/bin/php vendor/bin/phpunit tests/Unit/Configuration/MailConfigurationTest.php --no-coverage

Focused result: 1 test, 4 assertions.

I also ran the full backend unit suite. It completed with the existing NoInternalUrlRuleTest external URL failures, which are unrelated to this mail config change.