feat: Impact Monitor — AI impact evaluation for eco events & geopolitical news

- New DB tables: event_categories (18 bootstrap categories) + instrument_impacts
- impact_categories_bootstrap.py: FOMC/NFP/CPI/GDP/PCE/ISM/BOJ/ECB/BOE/OPEC+ + 7 géopolitical categories with per-instrument sensitivity & direction defaults
- impact_service.py: GPT-4o-mini evaluation (0-1 score, direction, rationale) + monitor data aggregation
- routers/impact.py: GET/POST endpoints for evaluate/bulk/monitor/adjust/categories
- ImpactMonitor.tsx: two-tab page (Évalués / À évaluer), top instruments bar, inline score override modal, category browser
- Startup auto-bootstrap for impact categories
- Route /impact + sidebar nav entry

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
+  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers, ScanEye, CandlestickChart, Crosshair
 } 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: '/impact', icon: Crosshair, label: 'Impact Monitor' },
   { to: '/snapshot', icon: ScanEye, label: 'Snapshot Externe' },
   { to: '/timeline', icon: Layers, label: 'Timeline' },
   { to: '/logs', icon: ScrollText, label: 'System Logs' },