feat: Refactor CLI API client by splitting billing, carrier, and charging modules into separate files

- Extract Invoice, Payment, Subscriber types and GetInvoices, GetPayments, GenerateInvoice methods to billing.go
- Extract CarrierInfo, ConnectivityStatus, ProfileInfo types and carrier connector methods to carrier.go
- Extract CreditCheckRequest/Response, CreditBalanceResponse types and charging engine methods to charging.go
- Add PacketStats, CreditInfo types and GetPacketStats, GetCreditInfo

apps/cli/internal/api/monitoring.go

@@ -0,0 +1,62 @@
+package api
+
+import "time"
+
+// Alert represents a monitoring alert
+type Alert struct {
+	ID       string    `json:"id"`
+	Severity string    `json:"severity"`
+	Service  string    `json:"service"`
+	Message  string    `json:"message"`
+	Time     time.Time `json:"time"`
+}
+
+// ListAlerts returns alerts
+func (c *Client) ListAlerts() ([]Alert, error) {
+	var resp struct {
+		Data []Alert `json:"data"`
+	}
+	if err := c.doGetJSON("/api/v1/alerts", &resp); err != nil {
+		return nil, err
+	}
+	return resp.Data, nil
+}
+
+// SystemStats represents system health/metrics
+type SystemStats struct {
+	ActiveSessions   int     `json:"active_sessions"`
+	TotalAccounts    int     `json:"total_accounts"`
+	BlockedUsers     int     `json:"blocked_users"`
+	LowBalanceAlerts int     `json:"low_balance_alerts"`
+	CPUUsage         float64 `json:"cpu_usage"`
+	MemoryUsage      float64 `json:"memory_usage"`
+	Uptime           string  `json:"uptime"`
+}
+
+// GetSystemStats retrieves system stats
+func (c *Client) GetSystemStats() (*SystemStats, error) {
+	var stats SystemStats
+	if err := c.doGetJSON("/api/v1/system/stats", &stats); err != nil {
+		return nil, err
+	}
+	return &stats, nil
+}
+
+// HealthStatus represents a health check result
+type HealthStatus struct {
+	Service   string    `json:"service"`
+	Status    string    `json:"status"`
+	Uptime    string    `json:"uptime"`
+	LastCheck time.Time `json:"last_check"`
+}
+
+// GetHealth returns per-service health
+func (c *Client) GetHealth() ([]HealthStatus, error) {
+	var resp struct {
+		Data []HealthStatus `json:"data"`
+	}
+	if err := c.doGetJSON("/api/v1/health", &resp); err != nil {
+		return nil, err
+	}
+	return resp.Data, nil
+}