feat(email): Integrate LeadMail API for transactional emails

Replace SMTP with LeadMail API service for sending system transactional emails (password resets, invitations, notifications). LeadMail provides built-in email verification via Verifalia and async queue-based sending.

Configuration:
- Set LEADMAIL_API_TOKEN and LEADMAIL_API_URL in .env
- Falls back to SMTP if LeadMail token not present
- Works via custom ActionMailer delivery method (stable across upstream merges)

Includes:
- LeadmailDelivery class with full spec coverage
- Support for multipart messages, attachments, CC/BCC
- Error handling and logging with message tracking
- Documentation in docs/LEADMAIL_INTEGRATION.md

.env.example

@@ -95,6 +95,11 @@ SMTP_OPENSSL_VERIFY_MODE=peer
 # SMTP_OPEN_TIMEOUT
 # SMTP_READ_TIMEOUT
 
+# LeadMail API (alternative to SMTP for transactional emails)
+# If LEADMAIL_API_TOKEN is set, it takes precedence over SMTP configuration
+# LEADMAIL_API_TOKEN=lm_your_token_here
+# LEADMAIL_API_URL=https://mail.leadmagnet.dev/api/v1
+
 # Mail Incoming
 # This is the domain set for the reply emails when conversation continuity is enabled
 MAILER_INBOUND_EMAIL_DOMAIN=