#860: Coerce PMP day values before scheduling

[gemshrine]

Fbe.regularly reads PMP day values from Factbase without coercing them. A string such as "28" reaches the Time subtraction as a string and raises TypeError; the interval value is also interpolated into the schedule query without numeric validation.

The method now converts both day values to whole numbers before querying or opening the factbase transaction. Invalid values raise Fbe::Error with the PMP property name. The default values and valid scheduling behavior remain the same.

Regression tests cover string interval and lookback values, repeated calls within the interval, and an invalid lookback value. The repository suite runs them with bundle exec rake test.

Fixes #860