refactor: move database-related projects to separate folder

SqlData/Domain/Schedule/Group.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+
+namespace Mirea.Api.DataAccess.Domain.Schedule;
+
+public class Group
+{
+    public int Id { get; set; }
+    public required string Name { get; set; }
+
+    public int? FacultyId { get; set; }
+    public Faculty? Faculty { get; set; }
+
+    public List<Lesson>? Lessons { get; set; }
+}