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

internal/db/dbq/scan_schedule.sql.go

@@ -0,0 +1,71 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: scan_schedule.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const getScanSchedule = `-- name: GetScanSchedule :one
+SELECT mode, interval_hours, time_of_day, weekly_day, updated_at
+  FROM scan_schedule WHERE id = true
+`
+
+type GetScanScheduleRow struct {
+	Mode          string
+	IntervalHours *int32
+	TimeOfDay     *string
+	WeeklyDay     *int32
+	UpdatedAt     pgtype.Timestamptz
+}
+
+// M7 recurring-scans: returns the singleton schedule row. Used by the
+// scheduler goroutine on boot and config-refresh, and by the admin GET
+// handler.
+func (q *Queries) GetScanSchedule(ctx context.Context) (GetScanScheduleRow, error) {
+	row := q.db.QueryRow(ctx, getScanSchedule)
+	var i GetScanScheduleRow
+	err := row.Scan(
+		&i.Mode,
+		&i.IntervalHours,
+		&i.TimeOfDay,
+		&i.WeeklyDay,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
+const updateScanSchedule = `-- name: UpdateScanSchedule :exec
+UPDATE scan_schedule
+   SET mode           = $1,
+       interval_hours = $2,
+       time_of_day    = $3,
+       weekly_day     = $4,
+       updated_at     = now()
+ WHERE id = true
+`
+
+type UpdateScanScheduleParams struct {
+	Mode          string
+	IntervalHours *int32
+	TimeOfDay     *string
+	WeeklyDay     *int32
+}
+
+// Admin PATCH replaces the whole row; CHECK constraints enforce validity.
+// The application normalizes mode='off' to NULL the per-mode fields
+// before calling this.
+func (q *Queries) UpdateScanSchedule(ctx context.Context, arg UpdateScanScheduleParams) error {
+	_, err := q.db.Exec(ctx, updateScanSchedule,
+		arg.Mode,
+		arg.IntervalHours,
+		arg.TimeOfDay,
+		arg.WeeklyDay,
+	)
+	return err
+}