Add Class Validation to Event Entity

[balintking]

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.