Add DeliveryTask, Payment, and Sale features

Implemented DeliveryTask, Payment, and Sale entities with full CRUD, business logic, and API endpoints. Updated Order/OrderItem for sale linkage and fulfillment tracking. Registered new DbSets and DI services. Revised documentation for delivery, sale, and payment workflows.

Distribution/Models/Enums.cs

@@ -42,4 +42,20 @@ namespace Distribution.Models
     public enum CancelRequestStatus { Pending, Approved, Rejected }
 
     public enum PurchaseOrderStatus { Issued, PartiallyReceived, Received, Closed, Cancelled }
+
+    public enum SaleStatus { Pending, Completed, Cancelled }
+
+    public enum OrderStatus { Pending, PartiallyFulfilled, Fulfilled, Cancelled }
+
+    public enum PaymentStatus { Pending, PartiallyPaid, Paid }
+
+    public enum PaymentMethod { Cash, Credit }
+
+    public enum DeliveryTaskStatus
+    {
+        Pending,
+        Delivered,
+        PartiallyDelivered,
+        Failed
+    }
 }