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/types.ts

@@ -757,6 +757,11 @@ export type RecentDevelopments = {
   weeklySnapshot: RecentDevelopmentsWeeklySnapshot | null;
 };
 
+export type PriceData<T> = {
+  value: T;
+  stale: boolean;
+};
+
 export type CompanyAnalysis = {
   company: {
     ticker: string;
@@ -766,10 +771,10 @@ export type CompanyAnalysis = {
     tags: string[];
     cik: string | null;
   };
-  quote: number;
+  quote: PriceData<number | null>;
   position: Holding | null;
-  priceHistory: Array<{ date: string; close: number }>;
-  benchmarkHistory: Array<{ date: string; close: number }>;
+  priceHistory: PriceData<Array<{ date: string; close: number }> | null>;
+  benchmarkHistory: PriceData<Array<{ date: string; close: number }> | null>;
   financials: CompanyFinancialPoint[];
   filings: Filing[];
   aiReports: CompanyAiReport[];