Add GET /api/cron/{id}/runs?limit=N endpoint that returns recent stored
runs for a cron job, with server-side limit clamping to 1-100 (default 20).
Frontend adds a CronRun type, API client function, and an expandable
run history panel on the Cron page showing status, timestamps, duration,
and output for each run, with loading, empty, error, and refresh states.
Closes#3299
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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