feat(score): rate limiting + focus trap + skip-link + touch targets + cache

SÉCURITÉ 8→9:
- lib/rate-limit.ts: Redis rate limiter (incr + expire)
- chat/route: 15 req/min max
- 6 routes IA: 20 req/min max (tags, title, labels, markdown, overview, summary)
- link-preview: cache Redis 5min par URL+userId
- auth-providers: rate limit login 5 req/5min

UX 7.5→8.5:
- search-modal: focus trap (Tab cycle + Shift+Tab)
- search-modal: ref modal pour querySelector focusable
- layout: skip-link 'Skip to content' (sr-only focus:not-sr-only)
- note-actions: boutons h-8 w-8 → h-9 w-9 (36px → touch target)

0 erreur TS, 199/199 tests

memento-note/app/api/chat/route.ts

@@ -15,6 +15,7 @@ import { trackFeatureUsage } from '@/lib/usage-tracker'
 import { readFile } from 'fs/promises'
 import path from 'path'
 import { logAuditEvent, getClientIp } from '@/lib/audit-log'
+import { rateLimit } from '@/lib/rate-limit'
 
 export const maxDuration = 60
 
@@ -53,6 +54,14 @@ export async function POST(req: Request) {
   }
   const userId = session.user.id
 
+  const rl = await rateLimit(userId, 'chat', 15, 60)
+  if (!rl.allowed) {
+    return new Response(JSON.stringify({ error: 'rate_limited' }), {
+      status: 429,
+      headers: { 'Content-Type': 'application/json', 'Retry-After': '60' },
+    })
+  }
+
   // GDPR AI Consent check
   if (!(await hasUserAiConsent())) {
     return new Response(JSON.stringify({ error: 'ai_consent_required' }), {