Five improvements to the /api/jobs endpoints:
1. Startup availability check — cron module imported once at class load,
endpoints return 501 if unavailable (not 500 per-request import error)
2. Input limits — name ≤ 200 chars, prompt ≤ 5000 chars, repeat must be
positive int
3. Update field whitelist — only name/schedule/prompt/deliver/skills/
repeat/enabled pass through to cron.jobs.update_job, preventing
arbitrary key injection
4. Deduplicated validation — _check_job_id and _check_jobs_available
helpers replace repeated boilerplate
5. 32 new tests covering all endpoints, validation, auth, and
cron-unavailable cases
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