nice refactorings

PracticeCalendar.Api/Controllers/ApiControllerBase.cs

@@ -0,0 +1,14 @@
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace PracticeCalendar.API.Controllers
+{
+    [ApiController]
+    [Route("api/[controller]")]
+    public class ApiControllerBase : ControllerBase
+    {
+        private ISender _mediator = null!;
+
+        protected ISender Mediator => _mediator ??= HttpContext.RequestServices.GetRequiredService<ISender>();
+    }
+}