feat(extensions): improve extension loading for backend modules

- Add load_extension_backend() to install in-memory modules and import entry points
- Entry points are module names that self-register on import
- Add volume mounts in docker-compose-light.yml for extensions development:
  - superset-core for local Pydantic models
  - extensions directory for .supx bundles
- Add EXTENSIONS_PATH config support
- Simplify init_extensions() to delegate to get_extensions()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/pythonpath_dev/superset_config.py

@@ -105,7 +105,15 @@ class CeleryConfig:
 
 CELERY_CONFIG = CeleryConfig
 
-FEATURE_FLAGS = {"ALERT_REPORTS": True}
+# Extensions configuration
+# For local development, point to the extensions directory
+# Note: If running in Docker, this path needs to be accessible from inside the container
+EXTENSIONS_PATH = os.getenv("EXTENSIONS_PATH", "/app/extensions")
+
+FEATURE_FLAGS = {
+    "ALERT_REPORTS": True,
+    "ENABLE_EXTENSIONS": True,
+}
 ALERT_REPORTS_NOTIFICATION_DRY_RUN = True
 WEBDRIVER_BASEURL = f"http://superset_app{os.environ.get('SUPERSET_APP_ROOT', '/')}/"  # When using docker compose baseurl should be http://superset_nginx{ENV{BASEPATH}}/  # noqa: E501
 # The base URL for the email report hyperlinks.