feat: Implement procurement services and number sequence management

- Added INumberSequenceService interface for generating document numbers.
- Created NumberSequenceService to handle atomic document number issuance.
- Introduced IPurchaseOrderService interface and implemented PurchaseOrderService for managing purchase orders.
- Added IRequisitionService interface and implemented RequisitionService for handling requisitions.
- Created IRfqService interface and implemented RfqService for managing RFQs and vendor quotations.
- Defined necessary DTOs and domain entities for procurement processes.
- Ensured proper validation and error handling across services.

Backend/ERPCore/Program.cs

@@ -48,6 +48,12 @@ builder.Services.AddScoped<ICategoryService, CategoryService>();
 builder.Services.AddScoped<IVendorService, VendorService>();
 builder.Services.AddScoped<IWarehouseService, WarehouseService>();
 
+// Cross-cutting + procurement services (docs/11 §3)
+builder.Services.AddScoped<INumberSequenceService, NumberSequenceService>();
+builder.Services.AddScoped<IRequisitionService, RequisitionService>();
+builder.Services.AddScoped<IRfqService, RfqService>();
+builder.Services.AddScoped<IPurchaseOrderService, PurchaseOrderService>();
+
 // Health checks (EF Core DB)
 builder.Services.AddHealthChecks().AddDbContextCheck<ErpDbContext>();