feat(server): retire scan scheduler for watcher + safety-net walk

Wire the fsnotify watcher and a fixed 12h safety-net delta walk in main;
remove the configurable scan scheduler (scheduler.go, scan_schedule table via
migration 0033, GET/PATCH /api/admin/scan/schedule, and the server/api
plumbing). Manual scan + scan status are unchanged.

internal/db/dbq/models.go

@@ -439,15 +439,6 @@ 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

@@ -1,71 +0,0 @@
-// 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
-}