feat: organize Grafana dashboards into folders

Enable sidecar folderAnnotation + foldersFromFilesStructure to group
26 dashboards into 5 managed folders:

- Cluster (6): k8s health, API server, nodes, pods, kube-state-metrics
- Networking (6): CoreDNS, Technitium, Headscale, ingress, network traffic
- Hardware (5): node-exporter, proxmox, iDRAC, UPS, NVIDIA GPU
- Operations (4): backup health, registry, audit logs, Loki
- Applications (2): realestate-crawler, qBittorrent

Dashboard-to-folder mapping defined in grafana.tf locals block.
External stacks (headscale, technitium) annotated individually.

stacks/headscale/modules/headscale/main.tf

@@ -433,6 +433,9 @@ resource "kubernetes_config_map" "grafana_headscale_dashboard" {
     labels = {
       grafana_dashboard = "1"
     }
+    annotations = {
+      grafana_folder = "Networking"
+    }
   }
   data = {
     "headscale.json" = file("${path.module}/dashboards/headscale.json")