feat: audit logging, secrets at rest, and legacy cleanup (#205)

* feat: audit logging, secrets at rest, and legacy cleanup

- Add Team Pro audit log: records all mutating API actions with user
  attribution, searchable timeline UI with filtering and pagination
- Add AES-256-GCM encryption at rest for node API tokens via CryptoService
- Drop 9 legacy SSH/TLS columns from nodes table (dead since v0.7)
- Remove orphaned MaintenanceModal.tsx (dead code, never imported)
- Add requireTeamPro backend guard for team-tier features
- Add audit log cleanup (90-day retention) to MonitorService
- Add docs page and navigation entry for audit log feature

* fix: remove unused AUTH_TAG_LENGTH constant from CryptoService

backend/src/services/MonitorService.ts

@@ -304,6 +304,7 @@ export class MonitorService {
             db.cleanupOldMetrics(isNaN(retentionHours) ? 24 : retentionHours);
             const retentionDays = parseInt(settings['log_retention_days'] || '30', 10);
             db.cleanupOldNotifications(isNaN(retentionDays) ? 30 : retentionDays);
+            db.cleanupOldAuditLogs(90);
         } catch (e) {
             console.error('MonitorService: failed to cleanup old data', e);
         }