Interface with Zapier

zapier has an interesting approach:

1. zapier calls a "subscribe" endpoint that the proca-sync provides with a rest url to call
2. proca-sync calls that rest url for every new message (normal)
3. zappier can call an "unsuscribe" end point that the proca-sync provides

https://platform.zapier.com/docs/triggers (push triggers)

just trying to flag now to see how/if it would fit in the architecture

Start with Zapier's push triggers documentation and inspect proca-sync's existing REST entry points, if any. The subscribe, callback, and unsubscribe flow needs an agreed architectural design before implementation can be considered done.

Written by the indexing model from the issue text.