🔧(backend) add setting to toggle application token exchange mechanism

Introduce a configuration flag to enable or disable the
application token exchange (service account) mechanism.

This allows activating alternative authentication backends
without requiring full application token configuration.

Required to support the upcoming add-ons authentication backend.

src/backend/core/external_api/viewsets.py

@@ -20,6 +20,7 @@ from rest_framework import (
 )
 
 from core import api, models
+from core.api.feature_flag import FeatureFlag
 from core.services.jwt_token import JwtTokenService
 
 from . import authentication, permissions, serializers
@@ -36,6 +37,7 @@ class ApplicationViewSet(viewsets.ViewSet):
         url_path="token",
         url_name="token",
     )
+    @FeatureFlag.require("application")
     def generate_jwt_access_token(self, request, *args, **kwargs):
         """Generate JWT access token for application delegation.