feat: initial datamodel + ef connection and migrations

Initial version of the data model
Includes EF initialization and migrations
EF migrations are applied on application start

TournamentOrganizer/ViewModels/MainViewModel.cs

@@ -0,0 +1,9 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace TournamentOrganizer.ViewModels;
+
+public partial class MainViewModel : ViewModelBase
+{
+    [ObservableProperty]
+    private string _greeting = "Welcome to Avalonia!";
+}