Introduce Alpha Vintage

[warlock20]

Known Issue: Yahoo Finance returns no historical data for many European stocks (.F, .DE, .SW, .AS tickers)

Impact:

• Periods where price data unavailable fall back to transaction cost_basis
• Results in portfolio value = cost for those periods
• Affects CAGR calculation accuracy when many holdings lack data

Observed Examples:

• R6C.F, GAZ.F, TWR.F, TATB.F - no data available
• Multiple European tickers with sparse or missing historical data

Current Mitigation:

• Fallback to cost_basis when historical price unavailable
• Nearest date matching within 7-day window for weekends/holidays
• Comprehensive logging to track data sources

Potential Solutions (Future):

• Integrate alternative data provider (Alpha Vantage, Polygon.io, etc.)
• Manual price data entry for critical holdings
• Transaction-based approximation (calculate from realized G/L patterns)
• Multi-provider fallback strategy (try Yahoo → fallback to Alpha Vantage)