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/lib/ai/providers/anthropic.ts

@@ -12,7 +12,12 @@ export class AnthropicProvider implements AIProvider {
    */
   constructor(apiKey: string, modelName: string = 'claude-sonnet-4-20250514', baseURL?: string) {
     const trimmedBase = baseURL?.trim().replace(/\/+$/, '');
-    const anthropicClient = createAnthropic(trimmedBase ? { apiKey, baseURL: trimmedBase } : { apiKey });
+    const zdrHeaders = { 'Anthropic-No-Train': '1' };
+    const anthropicClient = createAnthropic(
+      trimmedBase
+        ? { apiKey, baseURL: trimmedBase, headers: zdrHeaders }
+        : { apiKey, headers: zdrHeaders }
+    );
     this.model = anthropicClient.chat(modelName);
   }