Add FXMacroData forex data source

[roberttidball]

Summary

Adds FXMacroData as a daily FX reference-rate source in the existing ForexDataSource fallback chain.

The integration:

• fetches https://fxmacrodata.com/api/v1/forex/{base}/{quote} for 1D forex requests
• supports API keys via FXMACRODATA_API_KEY or FXMD_API_KEY
• converts FXMacroData daily reference rows into close-only kline rows with volume = 0
• falls through to the existing Twelve Data, Tiingo, and yfinance sources for unsupported/intraday requests
• adds config support through FXMacroDataConfig

FXMacroData provides official-source daily reference rates, so the adapter is intentionally limited to daily candles.

Validation

• python -m pytest tests\test_fxmacrodata_forex_source.py -q
• python -m compileall -q app\data_sources\forex.py app\config\data_sources.py tests\test_fxmacrodata_forex_source.py
• git diff --check