fix: add dbt-duckdb dep, drop non-existent prefect-dbt[duckdb] extra
prefect-dbt has no [duckdb] extra — that specifier was silently a no-op.
dbt-duckdb must be listed explicitly for the DuckDB adapter to be found.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>