Posts from JP connected sites without a JP paid plan are not indexed in tags feeds

Internal discussion ref: p1705052443959929-slack-C0180B5PRJ4

Expected behavior

When a user posts to a JP-connected site using a tag, the post should appear in that tag feed in the reader.

Actual behavior

When a user posts to a JP-connected site using a tag, the post appears in that tag feed in the reader only if there is an active JP paid plan on that site.

This is due to some filtering in place in the backend. Especially thinking of the prompts "View all answers" scenario, that shows posts for the daily prompt tag, we might consider showing a nudge "to educate the user that their blog can be included in more places with a plan".

(NOTE: if we go this way, for it to be reliable and future-proof, we should make it driven by a flag sent by the API)

Tested on [iPad 7th gen], iOS [17.0], Jetpack iOS / WordPress iOS [24.0]

Ref: Android twin issue

Start by reviewing the iOS reader tag-feed flow and the backend filtering described in the issue, then compare the Android twin issue #19997. Confirm where the JP paid-plan condition is applied and whether the API provides the proposed flag; done means eligible posts from JP-connected sites appear in tag feeds without requiring a JP paid plan.