Fix N+1 query on dashboard (workshop host)

[mroderick]

What

Adds :workshop_host to the eager_load in DashboardQuery.upcoming_events, resolving the eager-loading detection bullet logged for GET /.

Why

DashboardQuery.upcoming_events (app/queries/dashboard_query.rb) loaded workshop_host lazily on each workshop when rendering the front page, triggering an N+1 detected by Rails' eager-loading checker.

USE eager loading detected
  Workshop => [:workshop_host]
  Add to your query: .includes([:workshop_host])

The query already uses eager_load for :chapter, :sponsors, :organisers, :permissions, so :workshop_host joins the same preload.

Verification

bundle exec rspec spec/queries/dashboard_query_spec.rb — 9 examples, 0 failures.