Premise: After solving the mystery of the permanent token access, I have been able to post on my Readerly.Ink page from my app.
Problem: My app, however, is supposed to
Or do I need to create a different app for each page?
No, you don't need to do that. An app is still able to manage multiple pages.
It used to be that you just granted an app access to manage all pages you have admin access to. But since that is obviously a bit broad and risky, Facebook has since changed things - users can specify which of their pages they actually want an app to have access to.
You get asked for that on the login dialog, when the permission is initially requested - but that list does not auto-update later to include any pages you created after that happened.
Currently, there does not seem to be any more practical procedure to get the new pages included, than removing the granted permission, and then asking for it again - upon which the user will be asked to chose which of their current set of pages they want to grant access to again.
For a quick fix, you can use Graph API Explorer - use the "get token" functionality, remove the granted manage_pages
permission, and then ask for it again.
For a public facing app that has other people manage their own pages, probably such a step should be implemented within the app - remove the permission via API when necessary, then send them through the login flow again. (Might need to use the reauthenticate
parameter at this point.)