Add new Backend structure with proper .NET 8 projects

src/Backend/DiunaBI.Core/Models/ProcessSource.cs

@@ -0,0 +1,15 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace DiunaBI.Core.Models;
+
+public class ProcessSource
+{
+    #region Relations
+    [Required]
+    public Guid LayerId { get; init; }
+    [Required]
+    public Guid SourceId { get; init; }
+    public Layer? Source { get; init; }
+    #endregion
+}