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

app/delivery_methods/leadmail_delivery.rb

@@ -0,0 +1,105 @@
+class LeadmailDelivery
+  attr_reader :settings
+
+  def initialize(settings)
+    @settings = settings
+  end
+
+  def deliver!(message)
+    payload = build_payload(message)
+    response = http_client.post('/emails/send', payload)
+
+    unless response.success?
+      raise LeadmailDeliveryError, "LeadMail API error: #{response.status} - #{response.body}"
+    end
+
+    response_data = JSON.parse(response.body)
+    message.header['X-LeadMail-Log-ID'] = response_data.dig('data', 'log_id')
+    Rails.logger.info("Email sent via LeadMail: log_id=#{response_data.dig('data', 'log_id')}, to=#{message.to.inspect}")
+    response
+  rescue StandardError => e
+    Rails.logger.error("LeadMail delivery failed: #{e.message}\nPayload: #{payload.inspect}")
+    raise
+  end
+
+  private
+
+  def build_payload(message)
+    {
+      from: extract_from(message),
+      to: extract_recipients(message.to),
+      cc: extract_recipients(message.cc),
+      bcc: extract_recipients(message.bcc),
+      reply_to: extract_reply_to(message),
+      subject: message.subject,
+      html_body: html_body(message),
+      text_body: text_body(message),
+      attachments: extract_attachments(message),
+      options: {
+        on_verification_failure: 'strip',
+        allow_disposable: false
+      }
+    }.compact
+  end
+
+  def extract_from(message)
+    from_address = message.from&.first
+    from_display = message[:from]&.display_names&.first
+
+    {
+      email: from_address,
+      name: from_display
+    }.compact
+  end
+
+  def extract_recipients(addresses)
+    return nil if addresses.blank?
+
+    Array(addresses).map { |email| { email: email } }
+  end
+
+  def extract_reply_to(message)
+    reply_to_address = message.reply_to&.first
+    return nil if reply_to_address.blank?
+
+    { email: reply_to_address }
+  end
+
+  def html_body(message)
+    return message.html_part.body.to_s if message.html_part.present?
+    return message.body.to_s if message.content_type&.start_with?('text/html')
+
+    nil
+  end
+
+  def text_body(message)
+    return message.text_part.body.to_s if message.text_part.present?
+    return message.body.to_s if message.content_type&.start_with?('text/plain')
+
+    nil
+  end
+
+  def extract_attachments(message)
+    return nil if message.attachments.blank?
+
+    message.attachments.map do |attachment|
+      {
+        filename: attachment.filename,
+        mime_type: attachment.mime_type || 'application/octet-stream',
+        content: Base64.strict_encode64(attachment.body.to_s)
+      }
+    end
+  end
+
+  def http_client
+    @http_client ||= Faraday.new(
+      url: settings[:api_url],
+      headers: {
+        'Authorization' => "Bearer #{settings[:token]}",
+        'Content-Type' => 'application/json'
+      }
+    )
+  end
+end
+
+class LeadmailDeliveryError < StandardError; end