Rolls up this pass's multi-site work: live catalog replication (spokes
stay synced after joining, not just a one-time snapshot), identical
agent-signing keys across sites, coordinated master promotion with real
old-master demotion, the UI to actually see and use any of it, and two
real bugs found only by live two-container testing (site-promote's dead
authorization check, and masterJoinKey/replicationPushToken leaking to
the browser via GET /api/site/config).
See CHANGELOG.md for the full list.