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/models/task.go

@@ -5,6 +5,7 @@ type Task struct {
 	AgentID            string      `json:"agent_id"`
 	AgentName          string      `json:"agent_name,omitempty"`
 	CustomerName       string      `json:"customer_name,omitempty"`
+	CustomerNumber     string      `json:"customer_number,omitempty"`
 	Name               string      `json:"name"`
 	Action             string      `json:"action"`
 	Status             string      `json:"status"`