Adds the singleton scan_schedule table holding the operator's
recurring-scan config. mode is the discriminator (off/interval/
daily/weekly); CHECK constraints enforce per-mode field validity
(interval_hours required for interval; time_of_day for daily and
weekly; weekly_day 1..7 for weekly). Seed row is mode='off'.
Two new queries: GetScanSchedule (read singleton; used by scheduler
boot + admin GET) and UpdateScanSchedule (admin PATCH; application
normalizes per-mode fields to NULL when mode='off').
