Scope desktop media accounts by the authenticated desktop client_id so
the same user logging in from Mac and Windows no longer sees a shared
account list. The list, identity lookup, upsert, patch, tombstone and
async health sink paths now all require matching client_id, and the
active uniqueness index moves from (workspace, platform, uid) to
(workspace, client, platform, uid).
Also strengthen the desktop client_id seed: when the OS machine id is
unavailable, fall back to a hashed fingerprint of stable hardware MAC
addresses before the random installation id, so the same hardware
keeps the same client across reinstalls.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
