feat: Timeline Navigator — contexte historique 3 temporalités COVID → aujourd'hui

- 2 nouvelles tables SQLite : market_events + timeline_context
- 32 événements historiques seedés (long/medium/short de feb 2020 à juin 2026)
- timeline_service.py : bootstrap, get_events_for_date, génération commentaires GPT-4o-mini
- /api/timeline router : GET /day/{date}, GET /events, POST /generate/{date}, POST /bootstrap
- Timeline.tsx : navigateur date avec strip visuel, 3 panneaux contextuels, catalogue d'événements
- Sidebar : entrée Timeline avec icône Layers

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
+  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers
 } from 'lucide-react'
 import { useGeoRiskScore, useAiStatus, usePortfolioSummary } from '../../hooks/useApi'
 import clsx from 'clsx'
@@ -27,6 +27,7 @@ const nav = [
   { to: '/calendar', icon: Calendar, label: 'Calendar' },
   { to: '/institutional', icon: Building2, label: 'Inst. Reports' },
   { to: '/specialist-desks', icon: Users, label: 'Specialist Desks' },
+  { to: '/timeline', icon: Layers, label: 'Timeline' },
   { to: '/logs', icon: ScrollText, label: 'System Logs' },
   { to: '/config', icon: Settings, label: 'Configuration' },
 ]