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/ai/auto-labels/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
 import { autoLabelCreationService } from '@/lib/ai/services'

memento-note/app/api/ai/convert-markdown/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
 import { markdownToHtml } from '@/lib/markdown-to-html'

memento-note/app/api/ai/notebook-summary/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
 import { notebookSummaryService } from '@/lib/ai/services'

memento-note/app/api/ai/search-overview/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
 import { searchOverviewService } from '@/lib/ai/services/search-overview.service'

memento-note/app/api/ai/tags/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server';
 import { auth } from '@/auth';
 import { contextualAutoTagService } from '@/lib/ai/services/contextual-auto-tag.service';

memento-note/app/api/ai/title-suggestions/route.ts

@@ -1,3 +1,4 @@
+import { rateLimit } from '@/lib/rate-limit'
 import { NextRequest, NextResponse } from 'next/server'
 import { runLaneWithBillingUser, willUseByokForLane } from '@/lib/ai/provider-for-user'
 import { getSystemConfig } from '@/lib/config'