The Explorer API has required SEMANTICA_API_KEY (X-API-Key header) since
v0.6.5, failing closed with 503 when unconfigured. Both the explorer
README security note and docs/explorer-setup.md still claimed there was
no built-in authentication.
Update both to describe the actual behavior: API-key enforcement,
the 503 fail-closed mode, and the explicit SEMANTICA_ALLOW_ANONYMOUS=true
opt-in for local development.
Fixes#1028