Add sorting to mass closure history page

[retlehs]

The mass closure history page is now paginated on production, so finding the biggest events means paging through chronologically. This adds a sort control matching the front page's, with two options: Most recent (default, unchanged behavior) and Most closures (plugin_count DESC, recency as tiebreaker).

• GET /closures?sort=closures — new whitelisted sort param; invalid values fall back to recent
• Sort select above the event list, styled like the front page's, auto-submits on change via a reusable data-autosubmit handler in app.js (sr-only submit button as no-JS fallback)
• Pagination links preserve the active sort
• GET /api/closures accepts the same param and echoes "sort" in the response
• /closures.md sibling supports it too (sort added to mdContentParams); the events heading notes the active sort
• RSS feed stays recency-ordered
• Tests cover both sort orders and the equal-count tiebreak

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