feat: sortable columns on jobs, failed jobs, and history (#78)

[eclectic-coding]

Summary

• sort_header_th helper added to ApplicationHelper — generates a <th> with aria-sort, direction indicator (↑/↓), and URL via a lambda; reused across all three views
• Jobs (?sort=class_name|queue_name|priority|created_at) — SQL ORDER via the joined solid_queue_jobs table; sort+direction threaded through tabs, period buttons, filter form, discard redirect, and bulk selection form
• Failed jobs (?sort=class_name|queue_name|created_at) — same pattern; references(:job) always applied for the LEFT OUTER JOIN
• History (?sort=class_name|queue_name|finished_at) — direct columns on solid_queue_jobs, no join required
• Invalid sort params fall back to the default (created_at / finished_at desc)

Test plan

• Jobs list: click Job Class, Queue, Priority, Enqueued At headers — sorting flips correctly; sort survives tab switch and period change
• Failed jobs list: click Job Class, Queue, Failed At headers — sorts correctly
• History list: click Job Class, Queue, Finished At headers — sorts correctly
• Duration column header (History) is intentionally non-sortable (cross-DB SQL complexity)
• bundle exec rake passes

Closes #78

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