- Add modules/core/local-domain-loopback.nix: systemd service and
activation script that write configured service domains to a
Sovran-managed block in /etc/hosts (127.0.0.1 / ::1) so requests
originating on this computer reach Caddy without NAT loopback.
- Import local-domain-loopback.nix in modules/modules.nix.
- server.py: add _validate_domain_value, _is_loopback_address,
_resolve_all_addresses, _trigger_hosts_update helpers.
- server.py: update _check_domain_reachable to use --resolve so
reachability is checked locally via Caddy, not via NAT loopback.
- server.py: update _evaluate_domain_checklist, api_services inline DNS
check, and api_domains_check to recognise loopback resolution as an
intentional local override rather than a DNS mismatch.
- server.py: call _trigger_hosts_update from api_domains_set after
saving a service domain so the /etc/hosts entry is applied immediately.
- Add app/tests/test_loopback_diagnostics.py with 47 tests covering
domain validation, loopback detection, diagnostic checklist logic,
composite health, and api_domains_check."
