Cache-first strategy in AlphaVantageService to avoid redundant API calls

[Copilot]

fetchSharePrices() always hit the Alpha Vantage API first, treating the repository purely as an error fallback — meaning every call incurred HTTP latency and counted against rate limits even for previously fetched data.

Changes

• Cache-first lookup: fetchSharePrices() now queries the repository before making any HTTP request. A cache hit returns immediately; the API is only called on a cache miss.

List<PriceData> cached = repository.loadPrices(symbol, startDate, endDate);
if (!cached.isEmpty()) {
    log.debug("Returning cached prices for symbol={} ({} to {})", symbol, startDate, endDate);
    return cached;
}
// only reaches Alpha Vantage if cache is empty
String json = fetchTimeSeriesDailyJson(symbol, outputSize);

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