Preload event associations and add pagination

[rsmithlal]

Summary

• eager load event categories and cover image attachments in index
• paginate draft, upcoming, and past event groups individually in the grouped index view
• add feature spec to verify event grouping on the index page
• add request specs covering event CRUD actions

Testing

• bundle install (fails: Your Ruby version is 3.2.3, but your Gemfile specified 3.4.4)
• bundle exec rubocop (fails: bundler: command not found: rubocop)
• bundle exec brakeman -q -w2 (fails: bundler: command not found: brakeman)
• bundle exec bundler-audit --update (fails: bundler: command not found: bundler-audit)
• bin/codex_style_guard (fails: bundler: command not found: rubocop)
• bin/ci (fails: bundler: command not found: rails)

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https://chatgpt.com/codex/tasks/task_e_689b7c3d13248321af2a78793a16fe70