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/components/layout/Sidebar.tsx

@@ -1,7 +1,7 @@
 import { NavLink } from 'react-router-dom'
 import {
   LayoutDashboard, Globe, BarChart2, FlaskConical,
-  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers, ScanEye, CandlestickChart, Crosshair, PlayCircle
+  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers, ScanEye, CandlestickChart, Crosshair, PlayCircle, Radio
 } from 'lucide-react'
 import { useGeoRiskScore, useAiStatus, usePortfolioSummary } from '../../hooks/useApi'
 import clsx from 'clsx'
@@ -28,6 +28,7 @@ const nav = [
   { to: '/institutional', icon: Building2, label: 'Inst. Reports' },
   { to: '/specialist-desks', icon: Users, label: 'Specialist Desks' },
   { to: '/instruments', icon: CandlestickChart, label: 'Instrument Snap.' },
+  { to: '/market-events', icon: Radio, label: 'Market Events' },
   { to: '/impact', icon: Crosshair, label: 'Impact Monitor' },
   { to: '/cycle-actions', icon: PlayCircle, label: 'Cycle Actions' },
   { to: '/snapshot', icon: ScanEye, label: 'Snapshot Externe' },