Implement Yahoo Finance API for monthly closing prices and exchange rates

[Copilot]

Adds client-side API layer for fetching monthly closing asset prices and live exchange rates via Yahoo Finance (free, no API key required).

New modules

• src/types/priceApi.ts — Types for price data, exchange rates, cache entries, and rate limit tracking
• src/utils/priceApi.ts — Monthly closing prices via Yahoo Finance chart API (/v8/finance/chart, interval=1mo)
• src/utils/exchangeRateApi.ts — Live FX rates via Yahoo Finance quote API (/v7/finance/quote) using EUR-based currency pairs (EURUSD=X, etc.)

Design

• Caching: In-memory — 24h for monthly prices, 6h for exchange rates
• Rate limiting: 500 req/day with daily reset
• FX fallback: Degrades to existing DEFAULT_FALLBACK_RATES when Yahoo is unavailable
• Error handling: Graceful degradation for missing data, network failures, invalid tickers

Usage

import { fetchMonthlyClosingPrices, fetchMultipleMonthlyPrices } from './utils/priceApi';
import { fetchExchangeRatesAsMap } from './utils/exchangeRateApi';

// Monthly closing prices for a single ticker
const result = await fetchMonthlyClosingPrices('MSFT', 12);
// result.prices: [{ ticker, date, open, high, low, close, volume }, ...]

// Batch fetch
const prices = await fetchMultipleMonthlyPrices(['VTI', 'BND', 'AAPL']);

// Live exchange rates, compatible with existing ExchangeRates type
const { rates, isUsingFallback } = await fetchExchangeRatesAsMap();
// rates: { EUR: 1.0, USD: 0.926, GBP: 1.163, ... }

Tests

34 new tests covering fetch logic, caching, rate limiting, error paths, and type validation.

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