Feature Request: bi-directional Signal integration

Why require an onion server when you could just send commands back to Haven through Signal to get what you want? Not only to retrieve all alerts (say you lost internet yourself and never got the alert on SIgnal, you could ask it to resend), but you could also have a ping command instead of a heartbeat if desired to make sure the thing is still alive as well take photos/audio on the fly, get GPS if the thing starts moving, etc. require password security if desired to make sure if your signal gets hijacked someone doesn't start receiving your notifications, etc. etc.

Review Haven's existing onion-server alert flow and its Signal notification path before defining the proposed bidirectional commands. Scope would need to specify supported commands, authentication, and handling for alerts, photos, audio, and GPS, with done criteria agreed before implementation.

Written by the indexing model from the issue text.