Add PostgreSQL-backed data store for production deployments

[michaelchu]

Summary

• Adds DataStore abstraction and PostgresStore implementation that stores options/stock data in proper PostgreSQL tables (options_data, stocks_data) instead of parquet files on disk
• Auto-selects backend via get_store(): PostgreSQL when DATABASE_URL is set, parquet files otherwise
• Adds optopsy-data import CLI command to import local parquet files into PostgreSQL
• All existing call sites updated from ParquetCache() to get_store()

Test plan

• All 1863 existing tests pass
• Linting and formatting pass
• Local dev without DATABASE_URL still uses parquet cache
• With DATABASE_URL set, data is stored/read from PostgreSQL
• optopsy-data import ~/.optopsy/cache/options/SPY.parquet imports into Postgres
• optopsy-data cache size reports row counts from Postgres

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