feat: overlay stock price on trend charts with right Y axis

- New GET /api/market/prices/{ticker} endpoint serving OHLCV data from
  market_snapshots, deduped by bar_timestamp
- New useMarketPrices hook in frontend
- Trend chart now shows price (purple line) on a right Y axis ($)
  alongside trend metrics (%) on the left Y axis
- Custom tooltip formats price as dollars, metrics as percentages
- Price line uses connectNulls for days with missing bar data

frontend/src/api/hooks.ts

@@ -245,6 +245,26 @@ export function useTrendHistory(params?: { ticker?: string; window?: string; lim
   return useGet<TrendSummary[]>(['trend-history', params], 'query', path);
 }
 
+export interface MarketPrice {
+  ticker: string;
+  close: number;
+  open: number;
+  high: number;
+  low: number;
+  volume: number;
+  bar_timestamp: number;
+  captured_at: string;
+}
+
+export function useMarketPrices(ticker: string | undefined, limit = 30) {
+  return useGet<MarketPrice[]>(
+    ['market-prices', ticker, limit],
+    'query',
+    `/api/market/prices/${ticker}?limit=${limit}`,
+    !!ticker,
+  );
+}
+
 export function useTrend(id: string | undefined) {
   return useGet<TrendSummary>(['trend', id], 'query', `/api/trends/${id}`, !!id);
 }