Heartbeat-first dispatch: retire WoL-as-default, add WaitingReboot

Every supported host runs vetting-reporter in-OS and heartbeats every
30s. WoL was never the thing that started vetting — the heartbeat
response's reboot_for_vetting command was. Firing WoL first only
crowded the run log with misleading diagnostics when the real failure
mode is "reporter isn't installed."

- StartRun 409s if the host hasn't heartbeated within 60s, pointing
  the operator at /register/quick.sh.
- Dispatcher re-checks LastSeenAt at dispatch time (run may sit in
  Queued long enough for the host to go offline); stale hosts mark
  the run Failed with failed_stage=dispatch instead of looping.
- New StateWaitingReboot + TriggerRebootCommanded capture the actual
  semantics. StateWaitingWoL kept as the hook point for a future
  manual-override button.
- Tile disables the Start button with a quick.sh tooltip when the
  host is offline, matching the server-side 409.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/web/templates/host_detail.templ

@@ -88,10 +88,12 @@ templ HostDetail(d HostDetailData) {
 			<section class="detail-section detail-actions">
 				<h2>Actions</h2>
 				<div class="detail-actions-row">
-					if canStart(d.Tile.Latest) {
+					if canStart(d.Tile) {
 						<form method="post" action={ templ.SafeURL(fmt.Sprintf("/hosts/%d/start", d.Tile.Host.ID)) } class="inline">
 							<button type="submit">Start vetting</button>
 						</form>
+					} else if canStartIfOnline(d.Tile.Latest) {
+						<button type="button" disabled title="host is not heartbeating — install the reporter via /register/quick.sh on the target host">Start vetting</button>
 					} else {
 						<button type="button" disabled>Run in flight</button>
 					}