Hash paired bearer tokens with SHA-256 before storing in config and
in-memory. When authenticating, hash the incoming token and compare
against stored hashes. Backward compatible: existing plaintext tokens
(zc_ prefix) are detected and hashed on load; already-hashed tokens
(64-char hex) are stored as-is.
Closes#58
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>