Stop substituting synthetic market data when providers fail

- Replace synthetic fallback in getQuote()/getPriceHistory() with null returns
- Add QuoteResult/PriceHistoryResult types with { value, stale } structure
- Implement stale-while-revalidate: return cached value with stale=true on live fetch failure
- Cache failures for 30s to avoid hammering provider
- Update CompanyAnalysis type to use PriceData<T> wrapper
- Update task-processors to track failed/stale tickers explicitly
- Update price-history-card UI to show unavailable state and stale indicator
- Add comprehensive tests for failure cases
- Add e2e tests for null data, stale data, and live data scenarios

Resolves #14

components/charts/interactive-price-chart.tsx

@@ -53,7 +53,7 @@ export function InteractivePriceChart({
   const shouldShowVolume = showVolume && filteredData.some(isOHLCVData);
 
   return (
-    <div ref={chartRef} className="w-full">
+    <div ref={chartRef} className="w-full" data-testid="interactive-price-chart">
       {showToolbar && (
         <ChartToolbar
           chartType={chartType}