Add support for HTML email templates

[MikaTech-dev]

Description

Add support for sending custom HTML-formatted email templates through the API while maintaining the Mikatech branding and footer.

Acceptance Criteria

• Users can send POST requests to an endpoint with custom HTML text
• Custom HTML is rendered as the email template instead of the standard one
• All custom email templates automatically include:
  • Mikatech logo at the top
  • "Generated by Mikatech-dev, for {websiteName}" footer text
  • Professional styling that matches the standard template
• HTML sanitization is implemented to prevent XSS attacks
• API validates HTML input for malicious content
• Custom templates support dynamic variable substitution (e.g., {userName}, {websiteName})
• Documentation is updated with usage examples

Implementation Details

• Create a new endpoint: POST /email/send-template
• Request body should accept:
  • to: recipient email address
  • subject: email subject
  • htmlTemplate: custom HTML template
  • variables: object with template variables
  • websiteName: the website name for the footer
• Inject Mikatech logo and footer into the template automatically
• Validate and sanitize HTML before rendering
• Return appropriate success/error responses

Testing

• Test with valid HTML templates
• Test with malicious HTML (should be sanitized)
• Test with missing required fields
• Test variable substitution
• Test logo and footer are always included
• Test email delivery with custom templates