feat: Add AMF client for session termination and subscriber status notifications

- Add AMFClient struct with base URL and HTTP client
- Add NewAMFClient with 30s timeout
- Add TerminateSession for subscriber session termination with reason and timestamp
- Add NotifySubscriberUpdate for subscriber status change notifications
- Add SessionTerminationRequest and SessionTerminationResponse models
- Add generateRequestID helper for request tracking

apps/api-server/internal/services/amf_client.go

@@ -0,0 +1,126 @@
+package services
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// AMFClient represents the Access and Mobility Management Function client
+type AMFClient struct {
+	baseURL    string
+	httpClient *http.Client
+}
+
+// NewAMFClient creates a new AMF client
+func NewAMFClient(baseURL string) *AMFClient {
+	return &AMFClient{
+		baseURL: baseURL,
+		httpClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+// SessionTerminationRequest represents a session termination request to AMF
+type SessionTerminationRequest struct {
+	IMSI      models.IMSI `json:"imsi"`
+	Reason    string      `json:"reason"`
+	Timestamp time.Time   `json:"timestamp"`
+}
+
+// SessionTerminationResponse represents the response from AMF
+type SessionTerminationResponse struct {
+	Success   bool   `json:"success"`
+	Message   string `json:"message"`
+	SessionID string `json:"session_id,omitempty"`
+}
+
+// TerminateSession terminates a subscriber session in the AMF
+func (a *AMFClient) TerminateSession(ctx context.Context, imsi models.IMSI, reason string) error {
+	req := SessionTerminationRequest{
+		IMSI:      imsi,
+		Reason:    reason,
+		Timestamp: time.Now(),
+	}
+
+	jsonData, err := json.Marshal(req)
+	if err != nil {
+		return fmt.Errorf("failed to marshal session termination request: %w", err)
+	}
+
+	url := fmt.Sprintf("%s/api/v1/sessions/terminate", a.baseURL)
+	httpReq, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewBuffer(jsonData))
+	if err != nil {
+		return fmt.Errorf("failed to create HTTP request: %w", err)
+	}
+
+	httpReq.Header.Set("Content-Type", "application/json")
+	httpReq.Header.Set("X-Request-ID", generateRequestID())
+
+	resp, err := a.httpClient.Do(httpReq)
+	if err != nil {
+		return fmt.Errorf("failed to send session termination request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("AMF returned status %d", resp.StatusCode)
+	}
+
+	var response SessionTerminationResponse
+	if err := json.NewDecoder(resp.Body).Decode(&response); err != nil {
+		return fmt.Errorf("failed to decode AMF response: %w", err)
+	}
+
+	if !response.Success {
+		return fmt.Errorf("AMF session termination failed: %s", response.Message)
+	}
+
+	return nil
+}
+
+// NotifySubscriberUpdate notifies AMF of subscriber status changes
+func (a *AMFClient) NotifySubscriberUpdate(ctx context.Context, imsi models.IMSI, status models.SubscriberStatus) error {
+	req := map[string]interface{}{
+		"imsi":      imsi,
+		"status":    status,
+		"timestamp": time.Now(),
+	}
+
+	jsonData, err := json.Marshal(req)
+	if err != nil {
+		return fmt.Errorf("failed to marshal subscriber update request: %w", err)
+	}
+
+	url := fmt.Sprintf("%s/api/v1/subscribers/update", a.baseURL)
+	httpReq, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewBuffer(jsonData))
+	if err != nil {
+		return fmt.Errorf("failed to create HTTP request: %w", err)
+	}
+
+	httpReq.Header.Set("Content-Type", "application/json")
+	httpReq.Header.Set("X-Request-ID", generateRequestID())
+
+	resp, err := a.httpClient.Do(httpReq)
+	if err != nil {
+		return fmt.Errorf("failed to send subscriber update request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("AMF returned status %d", resp.StatusCode)
+	}
+
+	return nil
+}
+
+// generateRequestID generates a unique request ID
+func generateRequestID() string {
+	return fmt.Sprintf("%d", time.Now().UnixNano())
+}