I'm currently able to do the following:
- start gargantext-server (as it hosts central exchange and
dispatcher, currently)
- start a websocket connection:
  websocat ws://localhost:8008/ws
- subscibe to a topic (in websocat):
  {"request": "subscribe", "topic": {"type": "update_tree", "node_id":
  15}}
- optionally subscibe to other node_ids or start other websocat's with
  different subscriptions (can be multiple)
- fire up
  cabal v2-run gargantext-central-exchange -- client
  This triggers a node_id: 15 and node_id: 16 notification to be sent

You can send your own notifications, e.g. with Python:
import json
import nanomsg as n
s = n.Socket(n.PUSH)
s.connect('tcp://localhost:5560')
s.send(json.dumps({'type': 'update_tree_first_level', 'node_id': 15}))

Also, removed EKG (I don't know how to make it work with WS)

This commit removes the need for a custom Opaleye fork by relying on
a newly-created `opaleye-textsearch` package following the design
outlined here:

https://github.com/tomjaguarpaw/haskell-opaleye/pull/563#issuecomment-1782794439