Allows each GHL sub-account to use a different Cast sender ID instead of
the global CAST_SENDER_ID default.
- store.TokenRecord gains a sender_id field (MongoDB)
- store.UpdateSenderID method to set it per location
- cast.Client.SendSMS accepts a senderID override param (empty = use
client-level default)
- webhook.processOutbound reads the location's sender_id from the token
record and passes it to Cast
- new admin handler: PUT /api/admin/locations/{locationId}/sender-id
protected by Authorization: Bearer <INBOUND_API_KEY>
Co-Authored-By: Paperclip <noreply@paperclip.ing>
