feat: Refactor SubscriberService to split monolithic service into modular files with method organization by domain

- Remove all method implementations from subscriber.go (CreateSubscriber/GetSubscriber/GetSubscriberByIMSI/UpdateSubscriber/SuspendSubscriber/TerminateSubscriber/ListSubscribers/terminateSubscriberSessions/GetAccount/ListInvoices/GetInvoice/ListRatingPlans/GetRatingPlan/ListAlerts/SearchSubscribers/ResolveAlert/CreateAlert and all other methods)
- Update SubscriberService struct comment

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

@@ -0,0 +1,102 @@
+package services
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"time"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// ListAlerts lists alerts with optional filtering by subscriber, severity, and resolved status.
+func (s *SubscriberService) ListAlerts(ctx context.Context, limit int, offset int, subscriberId *int, severity *models.AlertSeverity, resolved *bool) ([]*models.Alert, int64, error) {
+	query := s.db.DB.WithContext(ctx).Model(&models.Alert{})
+
+	if subscriberId != nil {
+		query = query.Where("subscriber_id = ?", *subscriberId)
+	}
+	if severity != nil {
+		query = query.Where("severity = ?", *severity)
+	}
+	if resolved != nil {
+		query = query.Where("resolved = ?", *resolved)
+	}
+
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	var alerts []*models.Alert
+	if err := query.Preload("Subscriber").Limit(limit).Offset(offset).
+		Order("timestamp DESC").Find(&alerts).Error; err != nil {
+		return nil, 0, err
+	}
+
+	return alerts, total, nil
+}
+
+// SearchSubscribers searches subscribers by name, MSISDN, or IMSI.
+func (s *SubscriberService) SearchSubscribers(ctx context.Context, query string, limit int) ([]*models.Subscriber, error) {
+	var subscribers []*models.Subscriber
+	searchPattern := "%" + query + "%"
+	if err := s.db.DB.WithContext(ctx).
+		Where("first_name ILIKE ? OR last_name ILIKE ? OR msisdn ILIKE ? OR imsi ILIKE ?",
+			searchPattern, searchPattern, searchPattern, searchPattern).
+		Limit(limit).
+		Find(&subscribers).Error; err != nil {
+		return nil, fmt.Errorf("failed to search subscribers: %w", err)
+	}
+	return subscribers, nil
+}
+
+// ResolveAlert marks an alert as resolved.
+func (s *SubscriberService) ResolveAlert(ctx context.Context, alertId string) (*models.Alert, error) {
+	alertID, err := strconv.ParseUint(alertId, 10, 32)
+	if err != nil {
+		return nil, fmt.Errorf("invalid alert ID: %w", err)
+	}
+
+	var alert models.Alert
+	if err := s.db.DB.WithContext(ctx).
+		Model(&alert).
+		Where("id = ?", alertID).
+		Update("resolved", true).
+		Update("resolved_at", time.Now()).
+		Error; err != nil {
+		return nil, fmt.Errorf("failed to resolve alert: %w", err)
+	}
+
+	if err := s.db.DB.WithContext(ctx).
+		Preload("Subscriber").
+		First(&alert, alertID).Error; err != nil {
+		return nil, fmt.Errorf("alert not found: %w", err)
+	}
+
+	return &alert, nil
+}
+
+// CreateAlert creates a new alert for a subscriber.
+func (s *SubscriberService) CreateAlert(ctx context.Context, req *models.CreateAlertRequest) (*models.Alert, error) {
+	alert := &models.Alert{
+		Type:         req.Type,
+		Severity:     req.Severity,
+		Message:      req.Message,
+		SubscriberID: req.SubscriberID,
+		Timestamp:    time.Now(),
+		Resolved:     false,
+	}
+
+	if err := s.db.DB.WithContext(ctx).Create(alert).Error; err != nil {
+		return nil, fmt.Errorf("failed to create alert: %w", err)
+	}
+
+	if err := s.db.DB.WithContext(ctx).
+		Preload("Subscriber").
+		First(alert, alert.ID).Error; err != nil {
+		return nil, fmt.Errorf("failed to retrieve created alert: %w", err)
+	}
+
+	return alert, nil
+}