Cover all REST handlers with httptest-based tests using real SQLite:
zones (list, get, create, update, delete), records (list, get, create,
update, delete with validation/conflict cases), requireAdmin middleware
(admin, non-admin, missing context), and utility functions (writeJSON,
writeError, extractBearerToken, tokenInfoFromContext).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
