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

docs/LEADMAIL_INTEGRATION.md

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+# LeadMail Integration
+
+Chatwoot is configured to use LeadMail API for sending transactional emails (password resets, invites, notifications, etc.).
+
+## Configuration
+
+Set these environment variables:
+
+```env
+LEADMAIL_API_TOKEN=lm_your_token_here
+LEADMAIL_API_URL=https://mail.leadmagnet.dev/api/v1
+```
+
+## How It Works
+
+1. **Priority**: If `LEADMAIL_API_TOKEN` is present, LeadMail is used. Otherwise, falls back to SMTP.
+2. **Async Processing**: LeadMail accepts emails asynchronously (202 Accepted) and queues them for background processing.
+3. **Email Verification**: Includes Verifalia verification to filter invalid/disposable emails (configurable via `on_verification_failure` option).
+4. **Message Tracking**: Each sent email receives a `log_id` stored in the `X-LeadMail-Log-ID` header for debugging.
+
+## Email Types Using This
+
+All transactional emails:
+- User account invitations
+- Password reset links
+- Account notifications (e.g., OAuth disconnects)
+- Digest emails
+- System-generated messages
+
+## Not Affected
+
+- **Email inbox replies**: Each email inbox can use its own SMTP credentials (configured per-inbox in dashboard)
+- **Conversation messages**: Only transactional system emails use LeadMail
+
+## Fallback Behavior
+
+If LeadMail API is unavailable:
+- Errors are logged with the failed payload
+- Delivery exceptions propagate (configured via `config.action_mailer.raise_delivery_errors`)
+- SMTP fallback is NOT automatic; you must not set `LEADMAIL_API_TOKEN` to use SMTP
+
+## Debugging
+
+Check logs for:
+```
+Email sent via LeadMail: log_id=12345, to=["recipient@example.com"]
+```
+
+Failures include the payload for investigation:
+```
+LeadMail delivery failed: [error message]
+Payload: {...}
+```
+
+## Testing
+
+The delivery method is fully spec'd in `spec/delivery_methods/leadmail_delivery_spec.rb`.
+
+Run tests:
+```bash
+bundle exec rspec spec/delivery_methods/leadmail_delivery_spec.rb
+```