dot down some notes wrt to push

doc/impl-thoughts/PUSH.md

@@ -0,0 +1,22 @@
+# Push Notifications
+ - we setup the app on the sygnal server, with an app_id (io.element.hydrogen.web), generating a key pair
+ - we create a web push subscription, passing the server pub key, and get `endpoint`, `p256dh` and `auth` back. We put `webpush_endpoint` and `auth` in the push data, and use `p256dh` as the push key?
+ - we call `POST /_matrix/client/r0/pushers/set` on the homeserver with the sygnal instance url. We pass the web push subscription as pusher data.
+ - the homeserver wants to send out a notification, calling sygnal on `POST /_matrix/push/v1/notify` with for each device the pusher data.
+ - we encrypt and send with the data in the data for each device in the notification
+ - this wakes up the service worker
+ - now we need to find which local session id this notification is for
+
+## Testing/development
+
+ - set up local synapse
+ - set up local sygnal
+ - write pushkin
+ - configure "hydrogen" app in sygnal config with a webpush pushkin
+ - start writing service worker code in hydrogen (we'll need to enable it for local dev)
+ - try to get a notification through
+
+## Questions
+
+    - do we use the `event_id_only` format?
+    - for e2ee rooms, are we fine with just showing "Bob sent you a message (in room if not DM)", or do we want to sync and show the actual message? perhaps former can be MVP.