Update Channel Route

Toggle enabled status for a channel route.

Single-channel enforcement: when enabling a channel, all other non-webchat routes for this user are automatically disabled so exactly one messaging channel is active at a time.

If the user has no route and no known identifier for this channel yet (fresh onboarding), the selection is persisted via preferred_channel only. The route row is created later when the identifier arrives, either via an inbound message (OSS) or via an explicit link call (premium). This keeps placeholder rows out of the database and avoids leaking the user's internal UUID into identifier-shaped UI fields.