Adds a v2 credential envelope next to the v1 tool-secret helpers:
AES-256-GCM, a master key derived once per process from
ENCRYPTION_SECRET_KEY, and a per-record key from HKDF over the owner's
id, which is also the associated data, so a blob moved onto another
user's row does not decrypt. The envelope names its key, so
ENCRYPTION_SECRET_KEY_PREVIOUS keeps old rows readable during a
rotation.
Log redaction now also covers token_info, tokens and client_info, and
the API warns at startup when the public default key is in use.
