Tighten query-count assertions in the upcoming_events_for_user specs #3094

The regression specs added in #3074 assert that query counts stay within +2 of the one-workshop baseline:

• spec/queries/dashboard_query_spec.rb:186 — expect(four_query_count).to be <= one_query_count + 2
• spec/queries/dashboard_query_spec.rb:205 — expect(four_workshop_queries).to be <= one_workshop_queries + 2

Measured at 6749624, both paths issue 6 queries at one workshop and 6 at four, so exact equality passes today: expect(four_query_count).to eq(one_query_count).

The +2 slack lets a constant extra query per path through, which is the shape a fallback from preload to a non-batched load takes. The .upcoming_events query-count test in the same file asserts exact equality (expect(four_cards).to eq(one_card)).

The bound was relaxed during #3074 after review round 1 called the exact assertion stricter than sibling specs, so this is a deliberate tradeoff, not an oversight. Tightening both assertions restores the guard the N+1 fix was meant to provide.

Test-only change, no production risk.