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/repos/company-overview-cache.test.ts

@@ -56,10 +56,10 @@ function buildAnalysisPayload(companyName: string): CompanyAnalysis {
       tags: [],
       cik: null
     },
-    quote: 100,
+    quote: { value: 100, stale: false },
     position: null,
-    priceHistory: [],
-    benchmarkHistory: [],
+    priceHistory: { value: [], stale: false },
+    benchmarkHistory: { value: [], stale: false },
     financials: [],
     filings: [],
     aiReports: [],