LiveKit Cloud rejects room-join tokens as revoked when their `nbf`
predates a participant revocation. Zed generated those LiveKit JWTs with
`nbf: 0`, so a fresh participant token minted after stale connection
cleanup could still appear older than the cleanup and leave a user
joined at the collab layer without audio or screen sharing. This sets
`nbf` to the issuance time for room-join tokens while leaving admin/API
tokens unchanged, and adds regression coverage at the token, mock
LiveKit, and channel rejoin layers.

Closes FR-83

Release Notes:

- Fixed calls getting stuck without audio or screen sharing after
restarting Zed and rejoining a channel.