feat: Minstrel backs itself up nightly to a folder or an S3 bucket, off until an admin switches it on (#6773)

Admin › Backups shows a verdict first (off, not set up, waiting, backed up,
overdue, failing since when), Back up now, Test destination and the latest
runs, with the settings beside them.

- backup.Service: one run at a time, bounded at 30 minutes; the schedule
  ticks every 5 minutes, catches up a slot missed while the server was down,
  and retries a failure after an hour and then after as long as it has been
  failing. Runs are recorded in backup_runs, pruned after 90 days; a run cut
  off by a shutdown is marked failed at the next start.
- A folder destination writes database/minstrel-<UTC stamp>.tar and refuses
  a folder that isn't there. An S3 destination stages the file, hashes it,
  and uploads it under database/ through a throttled reader (4 MiB/s by
  default, 0 for no limit).
- Test destination checks the form as typed and only lists.
- The bucket secret is sealed like every stored credential; backup_settings
  and backup_runs are left out of the backup itself.
- The panel polls only while a run is going, capped at the run limit plus
  five minutes.

The music switch named in the task is left to #6774, so no control ships
that does nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/dbq/models.go

@@ -305,6 +305,31 @@ type AuditLog struct {
 	CreatedAt pgtype.Timestamptz
 }
 
+type BackupRun struct {
+	ID         pgtype.UUID
+	Trigger    string
+	Status     string
+	StartedAt  pgtype.Timestamptz
+	FinishedAt pgtype.Timestamptz
+	Location   string
+	Bytes      int64
+	Error      string
+}
+
+type BackupSetting struct {
+	ID             bool
+	Enabled        bool
+	Destination    string
+	Folder         string
+	Endpoint       string
+	Bucket         string
+	AccessKey      string
+	SecretKey      string
+	RunHour        int16
+	UploadLimitKib int32
+	UpdatedAt      pgtype.Timestamptz
+}
+
 type CandidateArtistTag struct {
 	CandidateMbid string
 	Tag           string