feat: serve the registration for explicit disclosure

Closes ChainSafe/canton-extending-mainnet#42

Summary: Scan serves the RegisteredSynchronizer at GET /v0/domains/{domain_id}/registration as a ContractWithState, so a buying validator can disclose it on AmuletRules_BuyMemberTraffic; the ACS store is the only source of the created event blob, since update history drops it by design.