fix: render the correct host sponsor on events listing cards

[mroderick]

Fixes #2975.

Summary

Workshop cards on the events listing could show the wrong sponsor as the venue, or none at all. EventsController#load_events eager-loaded workshop_host: :sponsor together with :sponsors; both route through the workshop_sponsors table in one join, and the host scope then binds to the wrong table alias, so workshop_host came back nil or pointing at a non-host sponsor on a plan-dependent subset of cards.

The host is now preloaded instead of joined, the same pattern Listable.most_recent and the chapter controller already use. Sponsor and organiser lists on cards also render in name order now (previously join-row order), and the card fragment cache version bumps to v3 so cards cached with the wrong venue are not served after deploy.

Review notes

• The fix approach is the main thing to weigh: preload versus deriving the host from eager-loaded workshop_sponsors records (the issue's other candidate, which would also cut join fan-out). Preload is the smaller change and matches the existing pattern; the derivation approach would need Workshop#host or the presenter to read from loaded through records.
• app/queries/dashboard_query.rb eager-loads :workshop_host alongside :sponsors in one join — the same alias trap, left out of this PR as follow-up work.

Testing

New specs cover: the host resolves to the host sponsor for every workshop on a page, the full sponsor list is present, name ordering for sponsors and organisers, and sorting with a blank member name (member names are only required for active members).

Post-deploy validation

The venue on a card is the visible change: spot-check /events/past and /events/upcoming after deploy — each workshop card should name its host sponsor. The v3 cache key re-warms the card cache on first render. Watch Rollbar for ArgumentError from presenter sorting as the failure signal; nil-name records were the trigger class the nil-safe sort covers.