Add Class Validation to Event Entity

Description:
Ensure that the data for the Event entity is validated before it is processed. This involves adding class-validator decorators to the DTOs used for creating and updating events.

Tasks:

- [ ] Create DTOs for creating
- [ ] and updating events.
- [ ] Add appropriate validation decorators to each field in the DTOs. Example: `@IsString(), @IsNotEmpty(), @IsDateString(), @IsOptional(), @IsEnum()`
- [ ] Update event controller methods to use these DTOs.

Acceptance Criteria:

- DTOs with validation decorators are created for event creation and update.
- Event controller methods use these DTOs.
- Validation errors are properly handled and returned to the client.