Standardize service errors with ProblemDetails envelope

Endpoints/ResultsWorkflowService.cs

@@ -11,7 +11,7 @@ internal sealed class ResultsWorkflowService(AppDbContext db)
     {
         var appState = await db.AppState.AsNoTracking().FirstAsync();
         if (!appState.ResultsOpen)
-            return Results.BadRequest(new { error = "Results are locked until the admin enables them." });
+            return EndpointHelpers.BadRequestError("Results are locked until the admin enables them.");
 
         var phase = await EndpointHelpers.GetCurrentPhaseAsync(db, player.Id);
         if (phase != Phase.Results)