docs: align API key guide with published 1.6.521

[chen21019]

Summary

• Align the current API Key guide with the published Server 1.6.521 / Engine 0.183.337 / Web Console 1.6.181 pairing.
• Document the existing public/numeric Key audit ID compatibility and HTTP 101 handshake boundary.
• Preserve owner RBAC, immutable numeric installation tags and explicit incomplete-matrix caveats; leave historical release notes unchanged.

Validation

• Documentation-only change; no runtime, API, workflow or permission changes.
• git diff --check passed.
• Every repository-relative link in the updated guide resolves.
• Release versions and statements checked against the published 1.6.521 release note and live GitHub release metadata.