* fix(frontend): keep invalid cron schedules as custom
* test(frontend): cover inclusive cron bounds
* docs(scheduled-tasks): note cron preset bounds
The form keeps an out-of-range cron as custom; the guide now says which
numeric ranges a preset covers and that saving such an expression returns
422. CHANGELOG entries cover the fix.
* docs(scheduler): clarify invalid cron save validation
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Co-authored-by: Nan Gao <thunderggnn@gmail.com>