feat(4-5/4-6): audit logging + zero-data-retention headers

Audit Logging (story 4-6):
- Nouveau modèle AuditLog (userId, action, resource, metadata, ip, createdAt)
- Migration 20260529143000_add_audit_log appliquée
- lib/audit-log.ts : logAuditEvent (fire-and-forget) + logAuditEventAsync + getClientIp
- auth.ts : LOG LOGIN / LOGOUT / USER_CREATED sur chaque event NextAuth
- /api/chat : log AI_REQUEST avec tokens + byok flag dans onFinish
- /api/agents/run-for-note : log AI_REQUEST avec featureKey + noteId

Zero-data-retention (story 4-5):
- OpenAI provider : header OpenAI-No-Training: 1
- Anthropic provider : header Anthropic-No-Train: 1
- DeepSeek provider : header X-No-Train: 1

sprint-status: 4-5 et 4-6 → done

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

memento-note/prisma/schema.prisma

@@ -946,3 +946,18 @@ model FlashcardReview {
   @@index([cardId])
   @@index([reviewedAt])
 }
+
+model AuditLog {
+  id        String   @id @default(cuid())
+  userId    String?
+  action    String
+  resource  String?
+  metadata  Json?
+  ip        String?
+  userAgent String?
+  createdAt DateTime @default(now())
+
+  @@index([userId])
+  @@index([action])
+  @@index([createdAt])
+}