feat: page Historique Positions + diff entre 2 snapshots PnL

Backend:
- get_pnl_snapshot(id) : détail complet d'un snapshot avec trades parsés
- diff_pnl_snapshots(a, b) : diff positions entre deux snapshots (nouvelles /
  fermées / évolution PnL par position + delta portfolio)
- GET /api/var/pnl/snapshots/{id} : détail snapshot
- GET /api/var/pnl/diff?a=&b= : calcul du diff

Frontend PositionHistory.tsx :
- Timeline scrollable des snapshots avec sparkline PnL
- Clic snapshot → détail des positions à ce moment (prix entrée, prix actuel,
  PnL %, PnL €, régime macro)
- Boutons A/B par snapshot → sélection de deux points à comparer
- Vue diff A→B : nouvelles positions, fermées, évolution PnL par trade,
  delta portfolio (capital, PnL %, PnL €)
- Route /position-history + nav sidebar

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
+  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare
 } from 'lucide-react'
 import { useGeoRiskScore, useAiStatus, usePortfolioSummary } from '../../hooks/useApi'
 import clsx from 'clsx'
@@ -21,6 +21,7 @@ const nav = [
   { to: '/analytics-advanced', icon: Microscope, label: 'Analytics Avancées' },
   { to: '/risk', icon: ShieldAlert, label: 'Risk Dashboard' },
   { to: '/var', icon: Gauge, label: 'VaR Analyse' },
+  { to: '/position-history', icon: GitCompare, label: 'Historique Positions' },
   { to: '/backtest', icon: History, label: 'Backtest' },
   { to: '/calendar', icon: Calendar, label: 'Calendrier' },
   { to: '/logs', icon: ScrollText, label: 'Logs Système' },