Compute available booking dates for a daily listing. Filters by bookable days of the week and excludes holidays. For listings with duration_days > 1, excludes start dates whose full range would hit a non-bookable day or extend past the booking window.

durationOverride lets callers compute availability for a span other than the listing's stored duration_days — used for "customisable days" listings, where duration_days is only the maximum, so the date list is built for a single day (every individually-bookable start) and the chosen span is validated separately at submit time via isBookingRangeValid.

Returns sorted array of YYYY-MM-DD strings.