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/app/api/chat/route.ts

@@ -12,6 +12,7 @@ import { checkEntitlementOrThrow, QuotaExceededError, incrementUsageAsync } from
 import { trackFeatureUsage } from '@/lib/usage-tracker'
 import { readFile } from 'fs/promises'
 import path from 'path'
+import { logAuditEvent, getClientIp } from '@/lib/audit-log'
 
 export const maxDuration = 60
 
@@ -433,6 +434,13 @@ Focus ONLY on this note unless asked otherwise.`
             trackFeatureUsage(userId, 'chat', final.usage?.totalTokens ?? 0)
             incrementUsageAsync(userId, 'chat')
           }
+          logAuditEvent({
+            userId,
+            action: 'AI_REQUEST',
+            resource: 'chat',
+            metadata: { tokens: final.usage?.totalTokens, byok: usedByok },
+            ip: getClientIp(req),
+          })
         },
       }),
   )