Add EF Core, Npgsql, Todo entity, and AppDbContext

backend/Models/Todo.cs

@@ -0,0 +1,10 @@
+namespace Backend.Models;
+
+public class Todo
+{
+    public long Id { get; set; }
+    public Guid UserId { get; set; }
+    public string Title { get; set; } = string.Empty;
+    public bool Completed { get; set; }
+    public DateTimeOffset CreatedAt { get; set; }
+}