Refactor data layer, add business layer

WebAPI/Controllers/NotFoundController.cs

@@ -0,0 +1,16 @@
+using Microsoft.AspNetCore.Mvc;
+
+namespace WebAPI.Controllers
+{
+    [ApiController]
+    [ApiExplorerSettings(IgnoreApi = true)]
+    [Route("error")]                 // ← required
+    public class NotFoundController : ControllerBase
+    {
+        [HttpGet("404")]             // ← required
+        public IActionResult Handle404()
+        {
+            return NotFound(new { message = "Route not found." });
+        }
+    }
+}