feat: recurring task list — enumerate tasks with next/last run and run-now action (#15)

[eclectic-coding]

Summary

• Adds GET /recurring_tasks — table of all SolidQueue::RecurringTask records showing key, cron schedule, job class or command, queue, next-run time, last-run time, and a static/dynamic badge
• Each row has a Run Now button that immediately enqueues the task via RecurringTasks::RunsController
• Adds Recurring link to the queue subnav (between Queues and History)
• Adds 4 realistic recurring tasks to the dev seed (3 static, 1 dynamic)
• Adds helper spec covering the format_duration minutes and hours branches
• Flash messages auto-dismiss after 6 seconds via a pure CSS keyframe animation (6s hold + 1s fade, layout collapses cleanly)

Test plan

• bundle exec rake passes (152 examples, 0 failures)
• GET /solid_stack/recurring_tasks renders the task table
• "Run Now" enqueues a task and redirects with a flash notice that auto-dismisses after ~6s
• Unknown task key redirects with "Recurring task not found" alert
• Empty state renders when no tasks are configured
• "Recurring" subnav link is active when on the recurring tasks page

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