fix(backup): seal backups and require step-up auth to restore

Backups carried every password hash, encrypted MFA secret and webhook
secret in plain JSON, and an import upserted arbitrary rows - including
users - so anyone holding backup:import, or tricking an admin into
importing a file, could overwrite the admin account with a known password.

- Export (download and scheduled) is AES-256-GCM over the gzipped bundle,
  key from BACKUP_ENCRYPTION_KEY or derived from JWT_ACCESS_SECRET via
  HKDF; header is authenticated, decompression is capped. Scheduled files
  are written 0600 in a 0700 directory; run-now returns only the file name.
- Import requires the current password (and TOTP when MFA is on).
- Only a sealed file restores users, roles, grants, webhooks and audit
  entries; an old unsealed file restores business data only and reports
  what it skipped.
- Audit entries are append-only on import.
- Imported documents: entityType checked against the enum, MIME type
  allowlisted and matched against the file signature, path kept under the
  storage root (no more directory traversal via entityType).
- Permission cache is cleared after an import.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/.env.example

@@ -38,6 +38,11 @@ BACKUP_CRON_SCHEDULE="0 3 * * *"
 # Wie viele der neuesten Backup-Dateien behalten werden, aeltere werden nach
 # jedem Lauf geloescht (Standard: 14).
 BACKUP_RETENTION_COUNT=14
+# Backups are encrypted (AES-256-GCM). Without this key they are sealed with a
+# key derived from JWT_ACCESS_SECRET and restore only into this deployment.
+# Set the same value on another deployment to restore there. At least 32
+# characters, e.g. "openssl rand -hex 32". Keep it outside the backup storage.
+# BACKUP_ENCRYPTION_KEY=""
 
 # Entra ID SSO. Ohne alle vier Werte bleibt SSO deaktiviert
 # (GET /api/v1/auth/sso/status meldet enabled: false, kein Fehler beim Start).