Stop serving credential secrets to administrators

Credential secrets - keypair secret keys, passwords, private keys - are never readable by any administrator (BEP-1077 5.3): administration is reissue and disable, and the secret is shown once to its owner at issuance. Close the legacy read paths that return another user's secret key to admins (legacy GraphQL keypair fields and any REST equivalent), keep self-reads, and make sure reissue/disable operations cover the administrative need. This is an intended narrowing - record it in the compatibility notes of the release that ships it.

Deferred: the right direction, but the frontend consumes the legacy secret-returning fields, so this lands in a later version coordinated with a WebUI release.

JIRA Issue: BA-7575

Start by locating the legacy GraphQL keypair fields and any REST equivalent, then check how the frontend consumes those secret-returning fields. Confirm that self-reads remain available and that reissue/disable cover administration; record the narrowing in the compatibility notes for the release coordinated with the WebUI version.

Written by the indexing model from the issue text.