Updated the `AddSQLExecutor<T>` method in the `DIExtensions` class to register `ISQLExecutor<T>` as a scoped service instead of a singleton. This ensures that a new instance of `SQLExecutor<T>` is created for each request within the same scope, improving resource management and request isolation.
Removed `DIExtensions.cs` and integrated its content into `DependencyExtensions.cs`, enhancing the dependency injection setup with repository and SQL executor registrations. Added a new `CreateEnvelopeSQL` class implementing the `ISQL<Envelope>` interface, currently with a placeholder implementation.