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

docs/docs.json

@@ -95,6 +95,7 @@
           "features/rbac",
           "features/atomic-deployments",
           "features/fleet-backups",
+          "features/audit-log",
           "features/licensing"
         ]
       },