fix(notifications): Send client_id when syncing read state

[nfebe]

Mark-as-read calls now forward the local client_id and use the existing POST endpoint, so notifications read offline can be linked to their server-side record on next sync instead of staying orphaned.

[austin047]

The thing is, client_id generation will run before mark as read, so if it has to work both the client_id and read_at will have to be optional. The first call will set the client id and then the second will mark it as read when the user opens the notification

Or we rename the endpoint to something else that reflects both of this actions