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/components/search-modal.tsx

@@ -372,11 +372,20 @@ export function SearchModal({ isOpen, onClose }: SearchModalProps) {
     }
   }
 
-  // Keyboard navigation
+  // Keyboard navigation + focus trap
+  const modalRef = useRef<HTMLDivElement>(null)
   useEffect(() => {
     if (!isOpen) return
     const handleKeyDown = (e: KeyboardEvent) => {
       if (e.key === 'Escape') { e.preventDefault(); onClose() }
+      else if (e.key === 'Tab' && modalRef.current) {
+        const focusable = modalRef.current.querySelectorAll<HTMLElement>('button, a, input, [tabindex]:not([tabindex="-1"])')
+        if (focusable.length === 0) return
+        const first = focusable[0]
+        const last = focusable[focusable.length - 1]
+        if (e.shiftKey && document.activeElement === first) { e.preventDefault(); last.focus() }
+        else if (!e.shiftKey && document.activeElement === last) { e.preventDefault(); first.focus() }
+      }
       else if (e.key === 'ArrowDown') { e.preventDefault(); setSelectedIndex(p => Math.min(p + 1, filteredMatches.length - 1)) }
       else if (e.key === 'ArrowUp') { e.preventDefault(); setSelectedIndex(p => Math.max(p - 1, 0)) }
       else if (e.key === 'Enter') {
@@ -412,6 +421,7 @@ export function SearchModal({ isOpen, onClose }: SearchModalProps) {
 
   return (
     <div
+      ref={modalRef}
       className="fixed inset-0 bg-black/40 dark:bg-black/60 backdrop-blur-sm flex items-center justify-center z-[200] p-4 sm:p-6 select-none"
       role="dialog"
       aria-modal="true"