add error handler middleware

Foxchat.Identity/Utils/OauthUtils.cs

@@ -1,3 +1,7 @@
+using Foxchat.Core;
+using Foxchat.Identity.Middleware;
+using Foxchat.Identity.Database.Models;
+
 namespace Foxchat.Identity.Utils;
 
 public static class OauthUtils
@@ -20,4 +24,11 @@ public static class OauthUtils
             return false;
         }
     }
+
+    public static Application GetApplicationOrThrow(this HttpContext context)
+    {
+        var token = context.GetToken();
+        if (token is not { Account: null }) throw new ApiError.Forbidden("This endpoint requires a client token.");
+        return token.Application;
+    }
 }