feat(stock): watchlist API 헬퍼 + useWatchlist 훅(낙관적 CRUD·알림) + 테스트

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHXzpsZQxKG9hQmNRfZjRS

src/api.js

@@ -852,3 +852,13 @@ export function compatPatchReading(id, body) {
 export function compatDeleteReading(id) {
   return apiDelete(`/api/saju/compat/readings/${id}`);
 }
+
+// ── Stock Watchlist / Trade Alerts (관심종목·매매 시그널) ──
+// GET  /api/stock/watchlist            → { watchlist: [{ ticker, name, note, params, added_at }] }
+// POST /api/stock/watchlist  body { ticker, name?, note? } → { ok: true }
+// DELETE /api/stock/watchlist/{ticker} → 200/404
+// GET  /api/stock/trade-alerts?days=N  → { alerts: [{ id, ticker, name, kind, condition, price, detail, fired_at }] }
+export const getWatchlist    = ()         => apiGet('/api/stock/watchlist');
+export const addWatchlist    = (body)     => apiPost('/api/stock/watchlist', body);
+export const removeWatchlist = (ticker)   => apiDelete(`/api/stock/watchlist/${encodeURIComponent(ticker)}`);
+export const getTradeAlerts  = (days = 7) => apiGet(`/api/stock/trade-alerts?days=${days}`);