Hub join should not downgrade an active member

Repeated Hub join requests should not downgrade active members to pending.

I found a membership edge case worth fixing. If a user is already an active member of a Hub, calling join again should be a no-op or should keep the member active. It should not recalculate the Hub policy and move that user back to pending. Expected behavior: active membership remains active when the same user repeats a join request. Actual risk: in a moderated or changed-policy Hub, a repeated join action can make a legitimate member unable to post until they are approved again. A safe fix would check the current membership first. If status is already active, return success with active status and do not overwrite it.