backup: enable timestamp chaining for old backup schedules

PTS timestamp chaining is controlled by a `chain_protected_timestamp_records` flag on the backup schedule:

```
SHOW SCHEDULES
... {"backup_statement":"BACKUP DATABASE backup_target INTO LATEST IN 'userfile:///backups/backup_target' WITH OPTIONS (detached)","backup_type":1,"chain_protected_timestamp_records":true,"dependent_schedule_id":1117366222981070800,"protected_timestamp_record":"829b152e-8b9d-49c2-8a04-4ac3584451ab"}
```

This is set automatically on new schedules. Any schedule created before this option was added will not have pts enabled.

We should consider backfilling this for pre-existing schedules. We could attach the backfill to full backups. When a full backup completes we have a valid PTS for the schedule and can use it to enable backup chaining.

When PTS chaining is not enabled, backups can fail with the following error:
```
Failed to run backup: running distributed backup to export ranges: exporting /Tenant//Table/: batch timestamp 1750910000.000000000
```

Jira issue: CRDB-55633

Epic CRDB-52076