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
-}

internal/db/migrations/0033_drop_scan_schedule.down.sql

@@ -0,0 +1,21 @@
+-- Recreate the scan_schedule singleton (mirror of 0019) so the drop is
+-- reversible. The application no longer reads or writes this table.
+CREATE TABLE scan_schedule (
+    id             boolean PRIMARY KEY DEFAULT true,
+    mode           text NOT NULL DEFAULT 'off',
+    interval_hours int,
+    time_of_day    text,
+    weekly_day     int,
+    updated_at     timestamptz NOT NULL DEFAULT now(),
+    CONSTRAINT scan_schedule_singleton CHECK (id = true),
+    CONSTRAINT scan_schedule_mode_check
+        CHECK (mode IN ('off', 'interval', 'daily', 'weekly')),
+    CONSTRAINT scan_schedule_interval_check
+        CHECK (mode != 'interval' OR (interval_hours IS NOT NULL AND interval_hours > 0)),
+    CONSTRAINT scan_schedule_daily_check
+        CHECK (mode != 'daily' OR time_of_day IS NOT NULL),
+    CONSTRAINT scan_schedule_weekly_check
+        CHECK (mode != 'weekly' OR (time_of_day IS NOT NULL AND weekly_day IS NOT NULL AND weekly_day BETWEEN 1 AND 7))
+);
+
+INSERT INTO scan_schedule (id) VALUES (true);

internal/db/migrations/0033_drop_scan_schedule.up.sql

@@ -0,0 +1,4 @@
+-- Retire the configurable scan scheduler. New-file pickup is now handled by
+-- the fsnotify watcher + a hardcoded safety-net delta walk (cmd/minstrel),
+-- so the operator-configurable schedule (and its admin UI) is gone.
+DROP TABLE IF EXISTS scan_schedule;

internal/db/queries/scan_schedule.sql

@@ -1,18 +0,0 @@
--- name: GetScanSchedule :one
--- M7 recurring-scans: returns the singleton schedule row. Used by the
--- scheduler goroutine on boot and config-refresh, and by the admin GET
--- handler.
-SELECT mode, interval_hours, time_of_day, weekly_day, updated_at
-  FROM scan_schedule WHERE id = true;
-
--- name: UpdateScanSchedule :exec
--- Admin PATCH replaces the whole row; CHECK constraints enforce validity.
--- The application normalizes mode='off' to NULL the per-mode fields
--- before calling this.
-UPDATE scan_schedule
-   SET mode           = $1,
-       interval_hours = $2,
-       time_of_day    = $3,
-       weekly_day     = $4,
-       updated_at     = now()
- WHERE id = true;