List email templates

Get a paginated list of email templates with filtering and sorting options. **Query Parameters:** - **page**: Page number (default: 1) - **limit**: Items per page (1-100, default: 10) - **search**: Search in name, description, and subject - **category**: Filter by template category - **isActive**: Filter by active status (true/false) - **tags**: Comma-separated list of tags to filter by - **sortBy**: Sort field (name, createdAt, updatedAt, category) - **sortOrder**: Sort direction (asc, desc) **Response:** - Paginated list of templates with usage statistics - Available categories and tags for filtering - Pagination metadata for navigation **Use Cases:** - Template management dashboard - Template selection for campaigns - Template library browsing - Usage analytics and reporting