Create email template

Create a new email template for reusable email notifications. **Features:** - HTML and plain text content support - Template variable system with {{variable}} syntax - Categorization and tagging - Validation and variable extraction - Metadata storage for custom properties **Template Variables:** - Use {{variableName}} syntax in subject, HTML, and text - Variables are automatically extracted and validated - Declare variables in the 'variables' array for better documentation - Unused or undeclared variables generate warnings **Categories:** - Organize templates by category (e.g., "marketing", "transactional", "system") - Default category is "general" - Categories help with filtering and organization **Use Cases:** - Welcome emails - Password reset notifications - Marketing campaigns - System alerts - Newsletter templates