fix: add backup safety preconditions - thin-pool headroom and tmpdir 1777

Add documented preflight checks for VM/CT backups on LVM thin-pool hosts:
- dmsetup status pve-data-tpool + lvs to verify data_percent < 90% and metadata_percent < 70%
- Exit 1 if pool shows Error/Fail state (takes down entire VG including host root)
- tmpdir must be mode 1777 (world-traversable) for vzdump archive step
- --output-format json for tasks started from truncating shells
- Document two incidents: acerpve thin-pool VM 101 (twice on 2026-09-13) and amdpve 0700 tmpdir (2026-09-14)
- Note metadata/snapshot-pressure hypothesis is UNPROVEN; preflight is the control
- Document GPU-host fact: VM 101 (llm-gpu) and VM 103 (ocu-llm) have no scheduled backup

infrastructure-control.prose.md

@@ -364,6 +364,68 @@ For docker-vm specifically:
 - No PBS backup in 48h → fail
 ```
 
+### Backup Safety Preconditions (2026-09-15)
+
+#### Background & Rationale
+
+Two incidents from 2026-09-13/14 demonstrate that backup operations can catastrophically fail when storage conditions are not verified first:
+
+1. **acerpve thin-pool VM 101** (acerpve, 2026-09-13): A snapshot-mode vzdump of VM 101 on acerpve filled the LVM thin pool. `dmsetup status pve-data-tpool` showed `thin-pool Error` (then `Fail`), the host root remounted `emergency_ro`, ordinary commands failed with I/O errors, LVM tools returned nothing and VM 101 (the RTX 3090 host) went unreachable while the host still answered ping and ssh. It happened TWICE in one day with different modes: snapshot at 13:39Z and a `--mode stop` cold run at 18:52Z. Both times a reboot rolled the failed transaction back and the pool returned rw (~30% data, ~1.2% metadata). Pool capacity was NOT the obvious explanation - ~816G with ~572G free - which is why the metadata/snapshot-pressure hypothesis stands unproven. A full or errored thin pool fails EVERY volume on the VG at once, including the host root.
+
+2. **amdpve 0700 tmpdir** (amdpve, 2026-09-14): A custom vzdump `tmpdir` created with mode 0700 broke a whole night of container backups: `fstat "<dir>/vzdumptmp<n>_<ct>//." failed - EACCES`, because the archive step runs through an unprivileged user namespace and could not traverse a root-owned 0700 directory. Fixed with `chmod 1777` (match /var/tmp) and proved with a real backup.
+
+3. **acerpve GPU-host fact**: VM 101 (llm-gpu) and VM 103 (ocu-llm) are in NO scheduled job, so their only coverage is one-off runs - and for VM 101 that is deliberate until the thin-pool is understood.
+
+#### PREFLIGHT Preconditions (Before ANY snapshot-mode backup on thin-pool hosts)
+
+Before starting ANY snapshot-mode vzdump on a host whose storage is an LVM thin pool, the following checks MUST pass:
+
+```bash
+# Check 1: Pool headroom
+dmsetup status pve-data-tpool | awk '{print $4, $6}'  # data_percent metadata_percent
+lvs -o lv_name,data_percent,metadata_percent,lv_size pve-data-tpool
+
+# Required thresholds (documented minimum):
+# data_percent < 90%  (80% recommended for safety margin)
+# metadata_percent < 70%  (metadata fills faster than data)
+
+# Check 2: Verify pool is not in error state
+dmsetup status pve-data-tpool | grep -q "Error\|Fail" && exit 1
+```
+
+**Minimum thresholds**: If either `data_percent >= 90%` or `metadata_percent >= 70%`, the backup MUST NOT start. State explicitly that these are hard stops, not warnings.
+
+**Why this is a precondition**: A full or errored thin pool fails EVERY volume on the VG at once, including the host root. This is not a soft failure - it takes down the entire Proxmox host.
+
+#### Staging Directory Requirement (2026-09-14 incident)
+
+Any custom vzdump `tmpdir` MUST be world-traversable and writable exactly like `/var/tmp` (mode 1777). The archive step of vzdump runs in an unprivileged user namespace and cannot traverse a root-owned 0700 directory.
+
+**Symptom to recognize**: `fstat "<dir>/vzdumptmp<n>_<ct>//." failed - EACCES` on every container in the backup run.
+
+**Fix**: `chmod 1777 <custom-tmpdir>` before starting vzdump.
+
+#### Task Start Rule for Truncating Shells
+
+When starting a backup task from a shell that may truncate output (e.g., pipes, `head`), always use:
+
+```bash
+pvesh create /storage/backup --output-format json -- ... | head -2
+# ❌ Can kill the backup task ("broken pipe" status)
+```
+
+Instead, capture JSON output without piping to truncating commands:
+
+```bash
+# Use --output-format json and capture to variable
+result=$(pvesh create /storage/backup --output-format json -- ...)
+# Then parse result if needed
+```
+
+#### GPU Host Backup Status (acerpve VM 101)
+
+VM 101 (llm-gpu) and VM 103 (ocu-llm) have NO scheduled backup job. Coverage is manual one-off runs only. This is intentional for VM 101 until the thin-pool failure mechanism is understood and documented.
+
 ## Section 5: Network Services — Monitoring
 
 ### 5.1 Service Inventory