Bus-level user allowlist (issue 0001a): the authoritative directory of
Ed25519 signing identities permitted to use the bus, independent of room
membership. Migration is additive and mirrored byte-for-byte between the
module-root migrations/ and the embedded pkg/membership/migrations/.
Store adds AddUser/GetUser/ListUsers/RevokeUser/IsAuthorized/HasAdmin.
IsAuthorized is the single fail-closed predicate both the control plane and
the NATS data plane will consult, so revocation is a status flip that denies
access on both without a restart.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
egutierrez
c5387028e0