Replication endpoints: the barrel_rep_transport callbacks as HTTP, under /db/:db/_sync/*.

These handlers call barrel_docdb directly (a documented layering exception: the replication machinery is deliberately absent from the barrel module); databases still resolve through barrel_server_dbs. Wire stance: JSON, base64 for HLCs and version vectors, version tokens as-is. Every request/response carries an x-barrel-hlc header for passive clock coupling.