Preload workshop_host in DashboardQuery.upcoming_events_for_user (per-workshop N+1 on /dashboard) #3070

DashboardQuery.upcoming_events_for_user runs one workshop_host lookup per workshop on every /dashboard render. Give it the same treatment as PR #3066: keep :sponsors in the join and add preload(workshop_host: :sponsor).

• app/queries/dashboard_query.rb:27-40 eager_loads :sponsors but never loads :workshop_host
• Workshop#host lazy-loads through workshop_host&.sponsor, so each card costs up to two extra queries — up to 2 × DashboardQuery::MAX_WORKSHOP_QUERY per render
• The dashboard action renders the same EventCardComponent → WorkshopPresenter#venue → model.host path that PR fix dashboard query by using .preload #3066 fixed for the homepage
• Do not add :workshop_host to the eager_load list: sharing the workshop_sponsors join alias is the Events listing can render the wrong sponsor as venue (eager_load host loss) #2975 host-loss trap

Raised from the review of PR #3066; the gap predates that PR.