Update email template
Update an existing email template.
**Features:**
- Partial updates supported (only provide fields to change)
- Automatic validation and variable extraction
- Version tracking with updatedAt timestamp
- Preserves creation metadata
**Validation:**
- Template content and structure validation
- Variable consistency checking
- Required field validation
- Warning generation for potential issues
**Use Cases:**
- Template content updates
- Configuration changes
- Bug fixes and improvements
- A/B testing variations
