Implement the two API endpoints documented in the README that were
previously returning 404:
- GET /api/v1/domains: Returns configured domains with their
discovered nameservers, last check time, and status (ok/pending).
- GET /api/v1/hostnames: Returns configured hostnames with
per-nameserver DNS records, status, and last check time.
Both endpoints read from the existing state store and config,
requiring no new dependencies. Nameserver results in the hostnames
endpoint are sorted for deterministic output.
Closes#67
