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

config/initializers/mailer.rb

@@ -27,11 +27,21 @@ Rails.application.configure do
   smtp_settings[:open_timeout] = ENV['SMTP_OPEN_TIMEOUT'].to_i if ENV['SMTP_OPEN_TIMEOUT'].present?
   smtp_settings[:read_timeout] = ENV['SMTP_READ_TIMEOUT'].to_i if ENV['SMTP_READ_TIMEOUT'].present?
 
-  config.action_mailer.delivery_method = :smtp unless Rails.env.test?
-  config.action_mailer.smtp_settings = smtp_settings
+  # Use LeadMail API if configured
+  if ENV['LEADMAIL_API_TOKEN'].present?
+    ActionMailer::Base.add_delivery_method :leadmail, LeadmailDelivery
+    config.action_mailer.delivery_method = :leadmail
+    config.action_mailer.leadmail_settings = {
+      api_url: ENV.fetch('LEADMAIL_API_URL', 'https://mail.leadmagnet.dev/api/v1'),
+      token: ENV['LEADMAIL_API_TOKEN']
+    }
+  elsif !Rails.env.test?
+    config.action_mailer.delivery_method = :smtp
+    config.action_mailer.smtp_settings = smtp_settings
 
-  # Use sendmail if using postfix for email
-  config.action_mailer.delivery_method = :sendmail if ENV['SMTP_ADDRESS'].blank?
+    # Use sendmail if using postfix for email
+    config.action_mailer.delivery_method = :sendmail if ENV['SMTP_ADDRESS'].blank?
+  end
 
   # You can use letter opener for your local development by setting the environment variable
   config.action_mailer.delivery_method = :letter_opener if Rails.env.development? && ENV['LETTER_OPENER']