feat: Hub monitoring takeover — event system, dead man's switch, notifications (v0.3.0)

Replace external Healthchecks.io with Hub-native monitoring. New events
table + /api/v1/event endpoint for structured events from controllers.
Staleness checker (60s) detects unresponsive nodes. Backup deadline
checker (daily 05:00) catches missed backups. Notification dispatcher
sends operator (English) + customer (Hungarian) emails via Resend with
per-event cooldowns. Event timeline on customer page, dashboard badges.
Config form deprecates Monitoring UUIDs section.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hub/internal/web/templates/dashboard.html

@@ -28,6 +28,7 @@
                 <tr>
                     <th>Customer</th>
                     <th>Status</th>
+                    <th>Events</th>
                     <th>Last Seen</th>
                     <th>CPU</th>
                     <th>Memory</th>
@@ -49,6 +50,7 @@
                             {{if eq .OverallStatus "ok"}}OK{{else if eq .OverallStatus "warn"}}WARN{{else if eq .OverallStatus "disabled"}}PAUSED{{else if eq .OverallStatus "pending"}}PENDING{{else}}DOWN{{end}}
                         </span>
                     </td>
+                    <td>{{if eq .OverallStatus "pending"}}—{{else}}{{if gt (add .EventErrors .EventWarnings) 0}}{{if gt .EventErrors 0}}<span class="severity-badge severity-error">{{.EventErrors}}</span>{{end}}{{if gt .EventWarnings 0}}<span class="severity-badge severity-warning">{{.EventWarnings}}</span>{{end}}{{else}}<span class="text-muted">—</span>{{end}}{{end}}</td>
                     <td>{{if eq .OverallStatus "pending"}}—{{else}}{{timeAgo .ReceivedAt}}{{end}}</td>
                     <td>{{if eq .OverallStatus "pending"}}—{{else}}{{formatFloat .CPUPercent}}%{{end}}</td>
                     <td>{{if eq .OverallStatus "pending"}}—{{else}}{{formatFloat .MemoryPercent}}%{{end}}</td>