Add od-backup-schedule runbook + od-backup-check script: schedule & monitor backups

Automate the nightly cold backup (Task Scheduler, filled-in local copy in
private tier), stagger the off-site upload, and MONITOR with three layers:
Task Scheduler last-run, a read-only health check (freshness/completeness/
size), and a dead-man's-switch heartbeat that pings an external monitor only
on success so silent failures and offline servers get caught. od-backup-check.ps1
is read-only (no DB/service), iex-safe, and pings <HEARTBEAT_URL> on PASS.
Cross-linked with od-db-backup.md and od-backup-verify.md; README updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -70,9 +70,11 @@ run time — never commit filled-in values.
 | [`od-scan-duplex.md`](od-scan-duplex.md) | Open Dental — duplex ADF scanner captures only one side (TWAIN, Show TWAIN UI branches) |
 | [`od-db-backup.md`](od-db-backup.md) | Open Dental — rock-solid cold backup of the database + images (stop/copy/start MySQL/MariaDB) |
 | [`od-backup-verify.md`](od-backup-verify.md) | Open Dental — verify a backup by test-restoring into an isolated Hyper-V VM (health checklist) |
+| [`od-backup-schedule.md`](od-backup-schedule.md) | Open Dental — schedule the backup + off-site upload and monitor it (dead-man's-switch heartbeat) |
 | [`scripts/cg-disable.ps1`](scripts/cg-disable.ps1) | Disable Credential Guard, then reboot (prompts to confirm) |
 | [`scripts/od-cfg-acl.ps1`](scripts/od-cfg-acl.ps1) | Grant Users Modify on FreeDentalConfig.xml (Option B of od-cfg-persist) |
 | [`scripts/od-db-backup.ps1`](scripts/od-db-backup.ps1) | Cold backup: stop MySQL/MariaDB, copy whole data dir + OpenDentImages, always restart (od-db-backup) |
+| [`scripts/od-backup-check.ps1`](scripts/od-backup-check.ps1) | Read-only backup health check: freshness/completeness/size + heartbeat ping (od-backup-schedule) |
 
 ## Tiers