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/frigate/main.tf

@@ -87,7 +87,7 @@ resource "kubernetes_deployment" "frigate" {
       }
       spec {
         node_selector = {
-          "gpu" : true
+          "nvidia.com/gpu.present" : "true"
         }
         toleration {
           key      = "nvidia.com/gpu"