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
- The message sender was incorrect. Incoming messages were created under the contact's name, and outgoing messages were created under the user's name instead of the reverse.
- The seed user's email address was incorrect in the message data.
- The Sendmail configuration overrode the Letter Opener config which made it difficult to test the email.
- This PR also fixes an ESLint lint issue on develop.
