Function revoke_member 

pub async fn revoke_member(
    storage: &dyn SyncStorage,
    cloud_home: &dyn CloudHome,
    chain: &mut MembershipChain,
    owner_keypair: &UserKeypair,
    revokee_pubkey: &str,
    timestamp: &str,
) -> Result<[u8; 32], InviteError>

Revoke a member from the library. This:

1. Revokes access on the cloud home
2. Creates a Remove membership entry signed by the owner
3. Generates a new library encryption key
4. Re-wraps the new key to all remaining members
5. Deletes the revoked member’s wrapped key
6. Uploads updated entries and keys

Returns the new encryption key (caller must persist it and start using it).