feat: Market Events admin page — source_refs + per-instrument impacts

- DB: colonne source_refs sur market_events (migration idempotente)
- save/update_market_event: persist source_refs (JSON array de {url,title,source})
- market_event_detector: stocke source_refs + évalue impacts instruments après chaque création
- Nouveau router /api/market-events: CRUD + evaluate + impacts CRUD
- Page MarketEvents.tsx: liste filtrée/triée + panneau détail (sources cliquables,
  tableau impacts par instrument avec score/direction/rationale inline-éditables,
  ajout manuel, bulk-evaluate)
- Sidebar: entrée Market Events (Radio icon)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -28,6 +28,7 @@ import ExternalSnapshot from './pages/ExternalSnapshot'
 import InstrumentDashboard from './pages/InstrumentDashboard'
 import ImpactMonitor from './pages/ImpactMonitor'
 import CycleActions from './pages/CycleActions'
+import MarketEvents from './pages/MarketEvents'
 import { Navigate } from 'react-router-dom'
 import { useCycleWatcher } from './hooks/useApi'
 
@@ -72,6 +73,7 @@ export default function App() {
             <Route path="/instruments" element={<Navigate to="/instruments/SPY" replace />} />
             <Route path="/instruments/:id" element={<InstrumentDashboard />} />
             <Route path="/impact" element={<ImpactMonitor />} />
+            <Route path="/market-events" element={<MarketEvents />} />
             <Route path="/cycle-actions" element={<CycleActions />} />
           </Routes>
         </main>