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/dbq/models.go

@@ -393,6 +393,15 @@ type ScanRun struct {
 	ArtistArtEnrich []byte
 }
 
+type ScanSchedule struct {
+	ID            bool
+	Mode          string
+	IntervalHours *int32
+	TimeOfDay     *string
+	WeeklyDay     *int32
+	UpdatedAt     pgtype.Timestamptz
+}
+
 type ScrobbleQueue struct {
 	ID            pgtype.UUID
 	UserID        pgtype.UUID