feat(db/m7-recurring-scans): migration 0019 + sqlc queries

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').

internal/db/migrations/0019_scan_schedule.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS scan_schedule;