The assert _db_path in fetch_analytics() would crash dashboard routes
locally when SERVING_DUCKDB_PATH is unset or serving.duckdb doesn't
exist yet. Change to graceful return [] so the app degrades cleanly.
Also add SERVING_DUCKDB_PATH=../serving.duckdb to local .env so the
web app will auto-connect once `materia pipeline run export_serving`
has been run for the first time.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>