Restructured project files across all layers:
- Removed `Controllers` folder reference from `DocumentOperator.API.csproj`.
- Added folder structure to `DocumentOperator.Application`, `DocumentOperator.Domain`, and `DocumentOperator.Infrastructure` projects for better organization.
Introduced new API configurations and middleware:
- Added `SerilogConfiguration` and `SwaggerConfiguration` classes.
- Added `ExceptionHandlingMiddleware`, `RequestLoggingMiddleware`, and `TenantResolutionMiddleware`.
Implemented new document processing feature:
- Added `ProcessDocumentCommand`, `ProcessDocumentHandler`, and `ProcessDocumentValidator` classes in the application layer.
Added `Asp.Versioning.Http`, `Microsoft.Extensions.Caching.StackExchangeRedis`, and `Serilog.AspNetCore` to `DocumentOperator.API` for API versioning, Redis caching, and structured logging.
Added `Ardalis.Result`, `FluentValidation`, `FluentValidation.DependencyInjectionExtensions`, and `MediatR` to `DocumentOperator.Application` for result handling, validation, and mediator pattern support.
Added `DevExpress.Pdf.Core` and `Microsoft.Extensions.Options.ConfigurationExtensions` to `DocumentOperator.Infrastructure` for PDF processing and configuration management.
