fix(config): reject non-positive SCHEDULER_ENTRY_FREQUENCY_FACTOR values

SCHEDULER_ENTRY_FREQUENCY_FACTOR was the only scheduler option without a
validator, so 0 was accepted at startup. With the entry_frequency polling
scheduler, the factor is used as part of a divisor in ScheduleNextCheck,
and a feed with weekly entries then triggered a division-by-zero panic
inside a background worker, crashing the daemon.

Require the factor to be >= 1, matching the validation of the other
scheduler options, so the misconfiguration fails at startup instead.