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/configs.go

@@ -223,12 +223,14 @@ func (s *Server) handleCustomerUnified(w http.ResponseWriter, r *http.Request, c
 		}
 	}
 
-	// History, notifications, infra backup
+	// History, notifications, events, infra backup
 	var history []store.CustomerSummary
 	var notifPrefs *store.NotificationPrefs
 	var recentNotifs []store.NotificationLogEntry
 	var infraMeta *store.InfraBackupMeta
 	var infraBackupAge string
+	var events []store.Event
+	var eventCounts map[string]int
 
 	if customer != nil {
 		history, _ = s.store.GetCustomerHistory(customerID, 24*time.Hour)
@@ -238,6 +240,8 @@ func (s *Server) handleCustomerUnified(w http.ResponseWriter, r *http.Request, c
 		if infraMeta != nil {
 			infraBackupAge = timeAgo(infraMeta.UpdatedAt)
 		}
+		events, _ = s.store.GetRecentEvents(customerID, 50)
+		eventCounts, _ = s.store.CountEventsBySeverity(customerID, time.Now().Add(-24*time.Hour))
 	}
 
 	type pageData struct {
@@ -270,6 +274,9 @@ func (s *Server) handleCustomerUnified(w http.ResponseWriter, r *http.Request, c
 		RecentNotifications []store.NotificationLogEntry
 		History             []store.CustomerSummary
 
+		Events      []store.Event
+		EventCounts map[string]int // severity → count (last 24h)
+
 		Flash     string
 		ActiveNav string
 	}
@@ -304,6 +311,9 @@ func (s *Server) handleCustomerUnified(w http.ResponseWriter, r *http.Request, c
 		RecentNotifications: recentNotifs,
 		History:             history,
 
+		Events:      events,
+		EventCounts: eventCounts,
+
 		Flash:     r.URL.Query().Get("flash"),
 		ActiveNav: "configs",
 	}
@@ -697,7 +707,7 @@ func buildConfigJSON(r *http.Request) string {
 		overrides["git"] = git
 	}
 
-	// Monitoring UUIDs
+	// Monitoring UUIDs (legacy — only written if user explicitly provides values)
 	uuids := make(map[string]interface{})
 	for _, key := range []string{"heartbeat", "system_health", "db_dump", "backup", "backup_integrity"} {
 		if v := strings.TrimSpace(r.FormValue("uuid_" + key)); v != "" {