test: integration test for msmtp email delivery

[smartwatermelon]

Context

The plex-watchdog feature uses msmtp to send email alerts. The BATS unit tests cover parsing and comparison logic but cannot test actual email delivery.

Scope

Evaluate and implement an integration test that verifies:

• msmtp can authenticate with Gmail SMTP
• Email is actually delivered (or at least accepted by the SMTP server)
• Error handling works when credentials are wrong or SMTP is unreachable

Considerations

• May require a test Gmail account or mock SMTP server
• Could use msmtp --serverinfo as a lighter-weight connectivity check
• Consider whether this should run in CI or only locally on the server

Related

• Part of the plex-watchdog feature
• app-setup/msmtp-setup.sh sends a test email during setup