feat: scheduled job management — run now, reschedule, run all now

[eclectic-coding]

Summary

• Adds ScheduledJobsController with standard CRUD: update (single job reschedule/run now) and create (bulk run all now via run_all_now collection route)
• Per-row actions on the scheduled jobs list: "Run Now" (back-dates to 1s ago), "+1h / +24h / +7d" offset buttons; Turbo Stream removes the row on run-now and replaces the scheduled_at cell on offset reschedule
• "Run All Now (N)" header button back-dates all scheduled executions matching the current period filter in one update_all call
• 18 request specs covering all offsets, Turbo Stream responses, error paths, period filter preservation, and plural/singular notice copy

Test plan

• POST /scheduled_jobs/run_all_now — back-dates all executions, updates job records, includes count in notice, pluralises, preserves period, rescue alert
• PATCH /scheduled_jobs/:id with offset=now — sets past time, turbo stream removes row
• PATCH /scheduled_jobs/:id with offset=1h/24h/7d — postpones by correct duration, turbo stream replaces cell
• Invalid offset — redirects with alert, does not mutate record
• Missing execution — redirects with alert
• Full suite: 121 examples, 0 failures

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