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/style.css

@@ -564,6 +564,55 @@ code {
     color: var(--text-muted);
 }
 
+/* Severity badges */
+.severity-badge {
+    display: inline-block;
+    padding: 0.15em 0.5em;
+    border-radius: 4px;
+    font-size: 0.8em;
+    font-weight: 600;
+    line-height: 1.4;
+}
+
+.severity-error {
+    background: rgba(239, 68, 68, 0.15);
+    color: #ef4444;
+}
+
+.severity-warning {
+    background: rgba(245, 158, 11, 0.15);
+    color: #f59e0b;
+}
+
+.severity-info {
+    background: rgba(59, 130, 246, 0.15);
+    color: #3b82f6;
+}
+
+/* Event filter buttons */
+.event-filter {
+    font-size: 0.8em;
+    padding: 0.2em 0.6em;
+    cursor: pointer;
+}
+
+.event-filter.active {
+    background: var(--accent);
+    color: #fff;
+    border-color: var(--accent);
+}
+
+/* Notification channel badges */
+.status-badge-operator {
+    background: rgba(139, 92, 246, 0.15);
+    color: #8b5cf6;
+}
+
+.status-badge-customer {
+    background: rgba(59, 130, 246, 0.15);
+    color: #3b82f6;
+}
+
 /* Responsive */
 @media (max-width: 768px) {
     .container { padding: 1rem; }