Standardize service errors with ProblemDetails envelope

Endpoints/EndpointHelpers.cs

@@ -1,5 +1,6 @@
 using GameList.Data;
 using GameList.Domain;
+using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
 using System.Security.Claims;
 
@@ -84,7 +85,28 @@ internal static class EndpointHelpers
     }
 
     public static IResult PhaseMismatch(Phase required, Phase current) =>
-        Results.BadRequest(new { error = $"This endpoint is available in the {required} phase. Your current phase is {current}." });
+        BadRequestError($"This endpoint is available in the {required} phase. Your current phase is {current}.");
+
+    public static IResult BadRequestError(string detail) => Problem(StatusCodes.Status400BadRequest, "Bad Request", detail);
+
+    public static IResult NotFoundError(string detail) => Problem(StatusCodes.Status404NotFound, "Not Found", detail);
+
+    public static IResult ConflictError(string detail) => Problem(StatusCodes.Status409Conflict, "Conflict", detail);
+
+    public static IResult UnauthorizedError(string detail = "Unauthorized") => Problem(StatusCodes.Status401Unauthorized, "Unauthorized", detail);
+
+    private static IResult Problem(int statusCode, string title, string detail)
+    {
+        return Results.Problem(
+            statusCode: statusCode,
+            title: title,
+            detail: detail,
+            extensions: new Dictionary<string, object?>
+            {
+                ["error"] = detail
+            }
+        );
+    }
 
     public static string? TrimTo(string? input, int max) =>
         string.IsNullOrWhiteSpace(input) ? null : input.Trim() is { Length: > 0 } t ? t[..Math.Min(t.Length, max)] : null;