fix: removed inject decorator provided by dependency_injector

[XyLearningProgramming]

Thank you for your amazing project showing an example of ddd pattern.

I found that the @injector decorators are actually not in use. For usages like Annotated[Application, Depends(get_application)], it is FastAPI that solves dependencies by calling this func directly: source code here.

If you'd like to use dependency-injector directly with FastAPI, according to this doc, the dependent should be formalized with dependency_injector.Provide as Depends(Provide[ApplicationContainer.application]) or Depends(Provide["application"]).

This PR removed @injector decorators.