add domain exception classes

BadRequestException, UnauthorizedException, ForbiddenException -
all extend DomainException. use cases throw these to signal HTTP
4xx categories; controllers translate to JsonResponse status
codes (400, 401, 403).

backend/app/Exceptions/BadRequestException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Exceptions;
+
+use DomainException;
+
+class BadRequestException extends DomainException {}

backend/app/Exceptions/ForbiddenException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Exceptions;
+
+use DomainException;
+
+class ForbiddenException extends DomainException {}

backend/app/Exceptions/UnauthorizedException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace App\Exceptions;
+
+use DomainException;
+
+class UnauthorizedException extends DomainException {}