Replace manual field extraction with EmbedObject for node logging
in gRPC handlers. Use zf.* constants for consistent field naming.
Changes:
- RegisterNode: use EmbedObject(node), zf.RegistrationKey, etc.
- SetTags: use EmbedObject(node)
- ExpireNode: use EmbedObject(node), zf.ExpiresAt
- RenameNode: use EmbedObject(node), zf.NewName
- SetApprovedRoutes: use zf.NodeID