Virtualization phase 2: ISOs, New VM from ISO, the VGA console in the browser, nightly exports, mounts, instance sensors

- Library: an ISO shelf in virt/isos: upload (the Files uploader), fetch by URL with an
  optional sha256, import a file from the machine by copying it in, rename, delete (refused
  while attached), attach to a VM as a CD-ROM live or stopped with boot-first, eject, and
  New VM from ISO: an empty VM with the ISO as first boot device, Secure Boot on or off.
- The VGA console: Ham Lab asks Incus for the VM's SPICE console over its unix socket and
  bridges the display websocket to the browser, where spice-html5 (vendored from upstream
  b372aab, LGPL, loaded on demand) draws it; Ctrl+Alt+Del, fit-to-window.
- Exports: every instance as a tarball in virt/exports/<name>/, kept to a count, as a cron
  line in /etc/cron.d/hamlab-virt calling hamlab virt-export, settings and the last run
  as files beside the data; Export now per instance or for all.
- Disks tab per instance: the root disk, attached ISOs with Eject, bind mounts in and out
  (shift for containers, virtiofs for VMs).
- instance.* sensors (running, down, state, cpu, mem) and the VM-or-container-down alert
  default, offered once to installs seeded before it existed.
- cloud-init: Use my SSH keys writes a cloud-config from the account's public keys.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/alerts.go

@@ -71,6 +71,9 @@ type alertSaved struct {
 	Channels []AlertChannel `json:"channels"`
 	Events   []AlertEvent   `json:"events"`
 	Seeded   bool           `json:"seeded"`
+	// SawInstanceRule marks that the instance-down default has been offered once; set so that
+	// removing it stays removed.
+	SawInstanceRule bool `json:"saw_instance_rule,omitempty"`
 	// Firing is what was firing when the hub last looked, so a restart (every deploy is one)
 	// carries on where it was and doesn't announce the same alerts all over again.
 	Firing map[string]alertKept `json:"firing,omitempty"`
@@ -102,6 +105,7 @@ var defaultRules = []AlertRule{
 	{ID: "machine-offline", Name: "Machine not answering", Sensor: "hamlab.reachable", Op: "==", Value: false, ForSec: 120, Severity: "crit", Enabled: true},
 	{ID: "container-down", Name: "Container down", Sensor: "container.*.down", Op: "==", Value: true, ForSec: 120, Severity: "crit", Enabled: true},
 	{ID: "container-unhealthy", Name: "Container unhealthy", Sensor: "container.*.health", Op: "==", Value: "unhealthy", ForSec: 180, Severity: "warn", Enabled: true},
+	{ID: "instance-down", Name: "VM or container down", Sensor: "instance.*.down", Op: "==", Value: true, ForSec: 120, Severity: "crit", Enabled: true},
 	{ID: "service-failed", Name: "A service has failed", Sensor: "services.failed", Op: ">", Value: 0.0, ForSec: 300, Severity: "warn", Enabled: true},
 	{ID: "disk-full", Name: "Filesystem nearly full", Sensor: "fs.*.used_pct", Op: ">=", Value: 90.0, ForSec: 300, Severity: "warn", Enabled: true},
 	{ID: "pool-unhealthy", Name: "ZFS pool not healthy", Sensor: "zfs.*.healthy", Op: "==", Value: false, ForSec: 60, Severity: "crit", Enabled: true},
@@ -122,6 +126,23 @@ func newAlertEngine(s *Server) *alertEngine {
 	for key, k := range e.saved.Firing {
 		e.state[key] = &alertState{pending: time.UnixMilli(k.Since), firing: true, since: time.UnixMilli(k.Since), value: k.Value}
 	}
+	// a default added after an install was seeded joins its rules once; removing it afterwards sticks
+	if e.saved.Seeded && !e.saved.SawInstanceRule {
+		has := false
+		for _, r := range e.saved.Rules {
+			if r.ID == "instance-down" {
+				has = true
+			}
+		}
+		if !has {
+			for _, d := range defaultRules {
+				if d.ID == "instance-down" {
+					e.saved.Rules = append(e.saved.Rules, d)
+				}
+			}
+		}
+		e.saved.SawInstanceRule = true
+	}
 	for i, r := range e.saved.Rules { // the first day's default watched "not running", which a container stopped on purpose also is
 		if r.ID == "container-down" && r.Sensor == "container.*.running" {
 			e.saved.Rules[i].Name, e.saved.Rules[i].Sensor, e.saved.Rules[i].Value = "Container down", "container.*.down", true