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/(main)/layout.tsx

@@ -41,7 +41,10 @@ export default async function MainLayout({
           <Sidebar user={session?.user} />
         </Suspense>
 
-        <main className="flex min-h-0 flex-1 flex-col overflow-y-auto scroll-smooth bg-memento-paper dark:bg-background">
+        <main className="flex min-h-0 flex-1 flex-col overflow-y-auto scroll-smooth bg-memento-paper dark:bg-background" id="main-content">
+          <a href="#main-content" className="sr-only focus:not-sr-only focus:absolute focus:z-[9999] focus:top-2 focus:left-2 focus:bg-brand-accent focus:text-white focus:px-4 focus:py-2 focus:rounded-lg focus:text-sm focus:font-bold">
+            Skip to content
+          </a>
           {children}
         </main>