Add Parquet/DuckDB export layer for market-inefficiency-hunter integration

[Sszmentor]

Goal

Bridge polymarket-mcp-server with market-inefficiency-hunter project. Currently zero integration surface (no polars, DuckDB, Parquet).

Requirements

1. Orderbook Snapshot Exporter

• New module: src/polymarket_mcp/export/parquet_writer.py
• Periodically call get_orderbook and write to Parquet
• Schema: (timestamp: datetime, market_id: str, token_id: str, side: str, price: f64, size: f64)
• Use polars for DataFrame construction

2. WebSocket → DuckDB Writer

• Attach callback to WebSocketManager.notification_callback
• Stream price_change and agg_orderbook events to DuckDB
• DuckDB can append to Parquet efficiently

3. Optional Dependencies

• Add [analytics] extras group in pyproject.toml: polars>=0.20, duckdb>=0.10, pyarrow>=15

4. New MCP Tools

• export_orderbook_snapshot(market_id, output_path) — one-shot Parquet export
• query_orderbook_history(market_id, start_ts, end_ts) — query DuckDB store

Context

• market-inefficiency-hunter uses polars (NOT pandas), DuckDB, Parquet everywhere
• See ~/Projects/AI_Agents/market-inefficiency-hunter/docs/decisions.md