✨(backend) accept form-urlencoded on the user token endpoint

Accept `application/x-www-form-urlencoded` requests on the user
token endpoint, in addition to the existing JSON support.

This aligns the endpoint with the OAuth 2.0 specification for token
endpoint requests (RFC 6749, sections 3.2 and 4.4.2), so standard
OAuth 2.0 client libraries can call it without any customization,
while keeping backward compatibility with existing JSON clients.

src/backend/core/external_api/viewsets.py

@@ -12,6 +12,9 @@ from rest_framework import decorators, mixins, viewsets
 from rest_framework import (
     exceptions as drf_exceptions,
 )
+from rest_framework import (
+    parsers as drf_parsers,
+)
 from rest_framework import (
     response as drf_response,
 )
@@ -41,6 +44,7 @@ class ApplicationViewSet(viewsets.ViewSet):
         methods=["post"],
         url_path="token",
         url_name="token",
+        parser_classes=[drf_parsers.FormParser, drf_parsers.JSONParser],
     )
     @FeatureFlag.require("application")
     def generate_jwt_access_token(self, request, *args, **kwargs):