[BACKEND] Home feed data — weekly top 5 carousel, ranking finetune, seen penalty

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The Home feed items from the bug bash that need server work, not styling.

• Weekly top 5 carousel. Home wants a carousel of the top 5 events of the week. Decide the source: a manual feature toggle on the event row, or derived from RSVP counts. A toggle is quicker and lets us hand-pick during beta; RSVP-derived is the real answer later. Probably ship the toggle first, with the column named so the derived version can replace it without a client change
• Recommendation ranking is too loose. It surfaces events on tags that are only distantly related to the user's interests. Tighten the scoring in the existing feed ranking
• Add a seen penalty. event_views already records (user_id, event_id), so an event a user has already scrolled past should score lower rather than reappearing at the top
• "Hosted by" org link is broken. Tapping the org under "Hosted by" opens a blank org page, and following fails with "could not update follow. Try again." Two separate problems most likely — the page not resolving the org id, and the follow endpoint erroring. Worth checking the server logs for the follow call first

The seen penalty is probably the highest-value of these — it's the difference between a feed that feels fresh and one that repeats.

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LOOP-273