Voice channel move: user disappears from list but audio still plays in original channel

This is closely related to #715 (voice channel move leaves the user stuck / LiveKit room transfer never fired server-side). That issue describes a disconnect on move; here the symptom is subtler: the UI moves the user, but the voice stream stays bound to the original channel's LiveKit room, so people in the original channel keep hearing them. Same root cause hypothesis: the destination channel's VOICE_SERVER_UPDATE / LiveKit token handoff never triggers. Worth fixing together. Repro (desktop, canary): User A in voice channel 1 → moderator with Move Members permission drags A to voice channel 2 → A disappears from channel 1 list, appears in channel 2, but A's audio is still heard in channel 1 and not in channel 2.