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/link-preview/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
+import { redis } from '@/lib/redis'
 
 export async function GET(req: NextRequest) {
   const session = await auth()
@@ -12,6 +13,16 @@ export async function GET(req: NextRequest) {
     return NextResponse.json({ error: 'Missing url' }, { status: 400 })
   }
 
+  const cacheKey = `linkpreview:${session.user.id}:${Buffer.from(url).toString('base64url').slice(0, 80)}`
+  try {
+    const cached = await redis.get(cacheKey)
+    if (cached) {
+      return NextResponse.json(JSON.parse(cached), {
+        headers: { 'Cache-Control': 'public, s-maxage=86400' },
+      })
+    }
+  } catch {}
+
   try {
     const parsed = new URL(url)
     if (!['http:', 'https:'].includes(parsed.protocol)) {
@@ -69,6 +80,8 @@ export async function GET(req: NextRequest) {
     const html = await res.text()
     const data = extractMetadata(html, parsed)
 
+    try { await redis.setex(cacheKey, 300, JSON.stringify(data)) } catch {}
+
     return NextResponse.json(data, {
       headers: { 'Cache-Control': 'public, s-maxage=86400' },
     })