Added ASP.NET backend with SQLite

backend/MuiCharts.Api/Controllers/ErrorController.cs

@@ -0,0 +1,19 @@
+using Microsoft.AspNetCore.Mvc;
+
+namespace MuiCharts.Api;
+
+/// <summary>
+/// Controller for handling errors.
+/// </summary>
+[ApiController]
+[Route("[controller]")]
+public class ErrorController: ControllerBase
+{
+	/// <summary>
+	/// Handles the HTTP GET request for the error endpoint.
+	/// </summary>
+	/// <returns>An IActionResult representing the error response.</returns>
+	[HttpGet]
+	[ProducesResponseType<ProblemDetails>(StatusCodes.Status500InternalServerError)]
+	public IActionResult Error() => Problem();
+}