- Introduced `HOP_ACME_STANDALONE_ONLY` env variable to run the ACME client without starting HTTP/DTLS servers.
- Allows certificate issuance/renewal solely and exits upon completion.
- Includes initialization of the ACME manager with domain verification, certificate management, and caching mechanisms. DomainService and expand Admin API
- Added `DomainServiceImpl` with support for registering, unregistering, and querying domains.
- Expanded Admin API with new endpoints:
- `GET /api/v1/admin/domains/exists` to check domain registration status.
- `GET /api/v1/admin/domains/status` to retrieve detailed domain information.
- Updated server initialization to wire `DomainService` and Admin API routes.
- Documented new Admin API endpoints in `API.md`.
