fix(dashboard): register Excel export task in docker dev CeleryConfig

The docker dev config overrides CELERY_CONFIG with its own imports tuple,
which was missing superset.tasks.export_dashboard_excel — exactly the
silent-failure mode documented in UPDATING.md: the endpoint returns 202
but the worker never registers the task, so exports never run in the
standard docker-compose dev environment.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/pythonpath_dev/superset_config.py

@@ -87,6 +87,7 @@ class CeleryConfig:
         "superset.tasks.scheduler",
         "superset.tasks.thumbnails",
         "superset.tasks.cache",
+        "superset.tasks.export_dashboard_excel",
     )
     result_backend = f"redis://{REDIS_HOST}:{REDIS_PORT}/{REDIS_RESULTS_DB}"
     worker_prefetch_multiplier = 1