feat: retry failed job with modified arguments

[eclectic-coding]

Summary

• Adds an editable arguments textarea to the job detail page for failed jobs
• Submitting the form updates job.arguments and calls execution.retry in one step
• Invalid JSON redirects back to the detail page with an alert, leaving the failed execution untouched
• All other job statuses continue to show the existing read-only <pre> display

Implementation

• FailedJobs::ArgumentsController#update — new controller following the existing nested-resource pattern (failed_jobs/arguments)
• Route: resource :arguments, only: [:update] nested inside resources :failed_jobs
• sqd-textarea CSS class added to _07_forms.css matching existing input/select styling
• 6 request specs; 100% line coverage maintained

Test plan

• Visit a failed job's detail page — Arguments card shows an editable textarea pre-filled with the current JSON
• Edit the JSON and click "Retry with these arguments" — redirects to /jobs/failed_jobs with a success notice; job appears in the ready queue with updated arguments
• Submit malformed JSON — redirects back to the detail page with "Invalid JSON" alert; failed execution remains
• Visit a non-failed job detail page — Arguments card still shows the read-only <pre>

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