feat: performance statistics page

[eclectic-coding]

Summary

• GET /stats — new StatsController aggregates all finished jobs by class name in Ruby (portable across SQLite, Postgres, MySQL) and computes execution count, avg, p50, p95, min, and max duration
• Sortable columns — every column header is a sort link toggling ?sort=&direction=; defaults to p95 descending so the slowest outliers surface first
• format_duration extended — now handles sub-second (Xms), fractional seconds (X.Xs with trailing .0 stripped via %g), minutes, and hours; fixes the pre-existing history spec that depended on whole-second output
• "Stats" link added to the queue subnav
• Sortable table header CSS — th a inherits muted colour, hover lifts to text colour, sort indicator sits inline

Test plan

• Visit /stats with no finished jobs — shows empty state
• Seed some jobs (bundle exec rake dev:reset) and visit /stats — table shows per-class rows with formatted durations
• Click column headers — sort order toggles asc/desc, active column shows arrow indicator
• Default load sorts by p95 descending
• bundle exec rake passes (167 examples, 0 failures)

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