Introduced a generic CreateCommandHandler class implementing MediatR's IRequestHandler to handle create commands. The handler uses a generic IRepository to perform asynchronous entity creation and supports dependency injection for repository access.
Introduced abstract UpdateCommand<TUpdateDto, TEntity> and UpdateCommandHandler<TCommand, TUpdateDto, TEntity> in EnvelopeGenerator.Application.Common.Commands. This provides a reusable, type-safe pattern for update operations using a generic repository and MediatR, requiring implementers to supply an update DTO and a query expression for entity selection.
