docs(disk-gc): clarify GC schedule applies to PBS datastore only

The 20:00 UTC PBS GC cron (proxmox-backup-manager datastore prune)
applies only to /tank/pbs-backup (pbs-datastore). It does NOT touch
media volumes (/media/*) which are report-only at all threat levels.

This clarification prevents the recurring confusion where a 96% media
volume triggers a GC expectation, when the GC schedule never applies
to it.

Closes the 2026-09-17 correction: 'the GC schedule is now 20:00 UTC,
protects the backup datastore, NOT the nearly-full media volume.'

disk-gc-threat-response.prose.md

@@ -228,6 +228,14 @@ call summary-reporter
   plan: plan
 ```
 
+## GC SCHEDULE (PBS datastore only)
+
+The PBS GC schedule `0 20 * * *` (20:00 UTC) applies **only** to the PBS datastore
+(`/tank/pbs-backup` on storepve), NOT to media volumes. Media volumes (/media/*) are
+report-only at all threat levels. The cron job runs `proxmox-backup-manager datastore
+prune --datastore storepve-datastore --keep-daily 35` which only affects the PBS
+datastore; it does not touch media volumes or any other filesystem.
+
 ## GC Strategies by Host Type
 
 ### Docker Hosts (kagentz 105, syslog-api 116, docker-vm 109, amdpve .15)