Fix PG::UndefinedColumn: column orders.product_id does not exist

[nbudin]

Summary

• Fixes PG::UndefinedColumn crash in ConventionReportsPresenter#sales_count_by_product_and_payment_amount
• product_id lives on order_entries, not orders — the bare symbol references in .group, .where.not, and .pluck were being resolved against the primary orders table
• Qualified all three references as "order_entries.product_id" / order_entries: { product_id: nil }
• Added a regression test that reproduces the original error and passes after the fix

Fixes #11217

Test plan

• Run bundle exec ruby -Itest test/presenters/convention_reports_presenter_test.rb — all 5 tests pass
• Visit the convention reports page on a convention with completed orders and verify the attendance by payment amount report loads without error

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/models/convention.rb	🟢 86.03%	🟢 86.76%	🟢 0.73%
app/presenters/convention_reports_presenter.rb	🟠 50%	🟠 58.7%	🟢 8.7%
Overall Coverage	🟢 52.62%	🟢 52.66%	🟢 0.04%

Minimum allowed coverage is 0%, this run produced 52.66%