Refactor registrant config and clean up duplicate endpoints

- Use REGISTRANT_* env vars instead of DEFAULT_* for registrant info
- Add REGISTRANT_ADDRESS2 to env config
- Remove duplicate /childns/* endpoints (keep /glue/* only)
- Add error handling to glue record endpoints
- Document /domains/register endpoint in README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -7,11 +7,12 @@ NAMECHEAP_SANDBOX=true
 # REST API Server Auth
 API_SERVER_KEY=your_api_server_key
 
-# Registrant Info
+# Registrant Info (default for domain registration)
 REGISTRANT_ORGANIZATION=
 REGISTRANT_FIRST_NAME=
 REGISTRANT_LAST_NAME=
 REGISTRANT_ADDRESS1=
+REGISTRANT_ADDRESS2=
 REGISTRANT_CITY=
 REGISTRANT_STATE_PROVINCE=
 REGISTRANT_POSTAL_CODE=