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

lib/server/company-analysis.ts

@@ -205,7 +205,7 @@ async function buildCompanyAnalysisPayload(input: {
     : null;
   const companyProfile = toCompanyProfile(secProfile, description);
   const valuationSnapshot = deriveValuationSnapshot({
-    quote: liveQuote,
+    quote: liveQuote.value,
     sharesOutstanding: secProfile?.sharesOutstanding ?? null,
     revenue: keyMetrics.revenue,
     cash: keyMetrics.cash,
@@ -238,10 +238,10 @@ async function buildCompanyAnalysisPayload(input: {
       tags: input.localInputs.watchlistItem?.tags ?? [],
       cik: latestFiling?.cik ?? null
     },
-    quote: liveQuote,
+    quote: { value: liveQuote.value, stale: liveQuote.stale },
     position: input.localInputs.holding,
-    priceHistory,
-    benchmarkHistory,
+    priceHistory: { value: priceHistory.value, stale: priceHistory.stale },
+    benchmarkHistory: { value: benchmarkHistory.value, stale: benchmarkHistory.stale },
     financials,
     filings: redactedFilings.slice(0, 20),
     aiReports,