Add DuckDB partitioned Parquet cleaning and export recipe #495

Context

As outlined in the Contributor Roadmap, DuckDB is widely used for analytical pipelines querying partitioned Parquet datasets. Providing a standalone recipe makes it easy for developers to clean and export partitioned datasets.

Task

Add a runnable example script in examples/integrations/duckdb_parquet_export.py showing how to read a partitioned Parquet dataset, clean it using FreshData, and write the cleaned result back out using DuckDB.

Acceptance Criteria

• Create examples/integrations/duckdb_parquet_export.py with standalone, executable code.
• Demonstrates creating/loading a multi-partition Parquet directory, executing fd.clean(), and exporting back to DuckDB table or Parquet destination.
• Document required dependency (pip install "freshdata-cleaner[duckdb]") in script docstring.
• Register the new recipe in examples/README.md.
• Script runs cleanly without errors via python examples/integrations/duckdb_parquet_export.py.

Pointers

• Reference existing DuckDB example: examples/integrations/duckdb_workflow.py
• Recipe index: examples/README.md

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