velero: implement three-tier backup retention policy

Replace single daily schedule (240h TTL) with three tiered schedules:
- daily-backup:   02:00 nightly,  192h TTL  → keeps last 7 daily backups
- weekly-backup:  03:00 Sundays, 1032h TTL  → keeps last 6 weekly backups
- monthly-backup: 04:00 on 1st,  4392h TTL  → keeps last 6 monthly backups

Kopia handles incremental deduplication at the block level automatically,
so each Velero backup only stores changed data despite being a full logical
snapshot. No additional incremental configuration required.

Update README with retention policy table and updated backup commands.

k8s/README.md

@@ -141,6 +141,25 @@ DATE=$( date +%Y%m%d )
 velero backup create $DATE --include-namespaces coder,home-assistant,n8n,influxdb --wait
 ```
 
+### Backup Retention Policy
+
+Three schedules run automatically, providing a tiered retention policy. Kopia (used for fs-backup) deduplicates at the block level, so each backup only stores changed data — incrementals come for free.
+
+| Schedule | Cron | TTL | Keeps |
+|---|---|---|---|
+| `velero-daily-backup` | `0 2 * * *` | 8 days | Last 7 daily backups |
+| `velero-weekly-backup` | `0 3 * * 0` | 43 days | Last 6 weekly backups (Sundays) |
+| `velero-monthly-backup` | `0 4 1 * *` | 183 days | Last 6 monthly backups (1st of month) |
+
+```sh
+# Check all schedules and their last run
+velero schedule get
+
+# Check current backups and their expiry
+velero backup get
+```
+
+
 ## coder
 
 Coder is a code server see [https://coder.com/docs/v2/latest/install/kubernetes](https://coder.com/docs/v2/latest/install/kubernetes).