feat: Task Health Check + customer_number in webhook payload

- Task model: CustomerNumber field (K-Nummer aus customers.number)
- DB: taskSelectCols joined c.number, scanTask scans CustomerNumber
- Hub: WaitForAgentOnline() polls until agent reconnects or timeout
- Scheduler: executeUpdateWithHealthCheck() waits for agent reconnect after update
  - health_check: {agent_online: bool, checked_at: RFC3339} in result
  - Status: completed_with_warning wenn Update OK aber Agent offline bleibt
- Frontend: Task-Tabelle zeigt Status-Badge (lauft/fehler/warnung/abgebr.)

backend/ws/hub.go

@@ -175,6 +175,19 @@ func (h *Hub) IsAgentConnected(agentID string) bool {
 	return exists
 }
 
+// WaitForAgentOnline polls until the agent reconnects or timeout is reached.
+// Returns true if agent came online within the timeout, false otherwise.
+func (h *Hub) WaitForAgentOnline(agentID string, timeout time.Duration) bool {
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if h.IsAgentConnected(agentID) {
+			return true
+		}
+		time.Sleep(10 * time.Second)
+	}
+	return false
+}
+
 func (h *Hub) GetConnectedAgents() []string {
 	h.agentsMux.RLock()
 	defer h.agentsMux.RUnlock()