- Add smtp.rs with send_email using lettre; supports none/starttls/smtps
- Replace use_tls: bool with TlsMode enum in SmtpConfig for explicit port 465 (SMTPS) support
- Add SMTP_IO_TIMEOUT (15s) for socket I/O and SMTP_WALL_TIMEOUT (30s) covering DNS + connect
- Spawn SMTP send on a dedicated thread so the IMAP worker thread is never blocked
- Update config.toml.example with tls_mode documentation
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
