BREAKING CHANGE: Removed FMP data fetcher and related configuration options
- Removed src/fmp.py module and all related tests
- Consolidated DataFetcherSingleton into a single implementation in src/stockdata.py
- Removed data_source configuration option from folio.yaml
- Simplified create_data_fetcher to only support YFinance
- Updated all imports and usages to use the simplified data fetcher
- Removed scripts/check_beta.py and tests/fetch_sample_data.py that depended on FMP
- Updated documentation to reflect YFinance as the only supported data source
This change simplifies the codebase by removing unnecessary abstraction and
focusing on a single data source implementation.
