docs/pbs-deployment: note ZFS case-insensitive + NFSv4 incompatibility

Discovered during Phase 1.3 (mount NAS datastore). The Debian NAS's
/mnt/backup is on ZFS with casesensitivity=insensitive; NFSv4 writes
fail with EACCES even for root with no_root_squash. Known-bad
combination at the ZFS-on-Linux + NFSv4 layer.

Workaround: mount the NFS share with vers=3. PBS's chunk-based
access pattern works fine over NFSv3.

Added a section 0.5 to Phase 0 documenting the issue + both fixes
(quick: use NFSv3; cleaner: create case-sensitive child dataset).
Future deployments against this NAS won't rediscover the same wall.

docs/runbooks/pbs-deployment.md

@@ -63,6 +63,36 @@ so Cockpit doesn't clobber it.
 Not blocking PBS work but worth tightening during a Cockpit pass —
 reduce to `10.0.0.0/8` or more specific. Open item, not blocker.
 
+### 0.5. ZFS case-insensitivity breaks NFSv4 writes
+
+The Debian NAS's `/mnt/backup` is a ZFS dataset with
+`casesensitivity=insensitive`. ZFS + NFSv4 + case-insensitive dataset
+is a known-bad combo: lookups/readdir work, `create`/`write` ops fail
+with EACCES even for root with `no_root_squash`.
+
+**Workaround:** mount with NFSv3 instead of NFSv4. Update the VM's
+`/etc/fstab` filesystem type to `nfs` and options to `vers=3`. PBS's
+access pattern (dedupe chunks are writen-once/read-many) works fine
+over NFSv3.
+
+Example `/etc/fstab` line that works:
+
+```
+10.250.50.50:/mnt/backup/pbs-ana  /mnt/pbs-datastore  nfs  defaults,_netdev,bg,hard,timeo=600,retrans=2,vers=3  0  0
+```
+
+**Cleaner long-term fix** (not required, but nicer): create a
+case-sensitive child dataset for PBS's data:
+
+```
+sudo zfs create -o casesensitivity=sensitive -o mountpoint=/mnt/backup/pbs-ds \
+     NASPool/backupStore/pbs-ds
+```
+
+Then export/mount `/mnt/backup/pbs-ds` via NFSv4 as originally planned.
+Case-sensitivity can't be changed on an existing dataset, which is why
+the parent is stuck on case-insensitive.
+
 ## Phase 1 — Provision PBS-ANA (VM on pfi-pve)
 
 **Target state:** a fresh Debian 12 VM on pfi-pve running