Add DuckDB as embedded analytics in Jupyter

[kiyeonjeon21]

Priority: P2

DuckDB is the "SQLite for analytics" — add it to Jupyter for in-process OLAP.

What to do

• Add duckdb to stacks/notebook/jupyter/requirements.txt
• Create notebooks/07-duckdb-analytics.ipynb
• Demonstrate: in-memory analytics, reading Parquet from MinIO, cross-source queries

Acceptance criteria

• DuckDB importable in Jupyter without errors
• Notebook demonstrates 3+ use cases
• No new Docker Compose profile needed (embedded in Jupyter)