gpu: schedule off NFD label, not k8s-node1 hostname

Remove every hardcoded reference to k8s-node1 that pinned GPU
scheduling to a specific host:

- GPU workload nodeSelectors: gpu=true -> nvidia.com/gpu.present=true
  (frigate, immich, whisper, piper, ytdlp, ebook2audiobook, audiblez,
  audiblez-web, nvidia-exporter, gpu-pod-exporter). The NFD label is
  auto-applied by gpu-feature-discovery on any node carrying an
  NVIDIA PCI device, so the selector follows the card.

- null_resource.gpu_node_config: rewrite to enumerate NFD-labeled
  nodes (feature.node.kubernetes.io/pci-10de.present=true) and taint
  each with nvidia.com/gpu=true:PreferNoSchedule. Drop the manual
  'kubectl label gpu=true' since NFD handles labeling.

- MySQL anti-affinity: kubernetes.io/hostname NotIn [k8s-node1] ->
  nvidia.com/gpu.present NotIn [true]. Same intent (keep MySQL off
  the GPU node) but portable when the card relocates.

Net effect: moving the GPU card between nodes no longer requires any
Terraform edit. Verified no-op for current scheduling — both old and
new labels resolve to node1 today.

Docs updated to match: AGENTS.md, compute.md, overview.md,
proxmox-inventory.md, k8s-portal agent-guidance string.

stacks/k8s-portal/modules/k8s-portal/files/src/routes/agent/+server.ts

@@ -138,7 +138,7 @@ Kyverno auto-generates LimitRange + ResourceQuota per namespace based on tier la
 
 - **Proxmox**: 192.168.1.127 (Dell R730, 22c/44t, 142GB RAM)
 - **Nodes**: k8s-master (10.0.20.100), node1 (GPU, Tesla T4), node2-4
-- **GPU workloads**: \`node_selector = { "gpu": "true" }\` + toleration \`nvidia.com/gpu\`
+- **GPU workloads**: \`node_selector = { "nvidia.com/gpu.present" : "true" }\` + toleration \`nvidia.com/gpu\` (label auto-applied by gpu-feature-discovery, no hostname pins)
 - **Pull-through cache**: 10.0.20.10 — use versioned image tags (cache serves stale :latest manifests)
 - **MySQL InnoDB Cluster**: 3 instances on iSCSI
 - **SMTP**: \`var.mail_host\` port 587 STARTTLS