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_crud.go

@@ -0,0 +1,127 @@
+package services
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/database"
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// CreateSubscriber creates a new subscriber with allocated IMSI.
+func (s *SubscriberService) CreateSubscriber(ctx context.Context, req *CreateSubscriberRequest) (*models.Subscriber, error) {
+	imsi, err := s.allocateIMSI(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to allocate IMSI: %w", err)
+	}
+
+	authKey, opc, err := s.generateAuthKeys()
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate auth keys: %w", err)
+	}
+
+	subscriber := &models.Subscriber{
+		IMSI:           imsi,
+		MSISDN:         req.MSISDN,
+		FirstName:      req.FirstName,
+		LastName:       req.LastName,
+		Email:          req.Email,
+		OrganizationID: req.OrganizationID,
+		Status:         models.SubscriberStatusActive,
+		PlanID:         req.PlanID,
+		AuthKey:        authKey,
+		OPc:            opc,
+		ServingPLMN:    models.PLMN{MCC: "208", MNC: "93"},
+		ProfileStatus:  models.ProfileStatusInactive,
+	}
+
+	if err := s.db.CreateSubscriber(ctx, subscriber); err != nil {
+		return nil, fmt.Errorf("failed to create subscriber: %w", err)
+	}
+
+	if req.EUICCID != "" {
+		subscriber.EUICCID = req.EUICCID
+		go func() {
+			bgCtx := context.Background()
+			if err := s.provisionESIMProfile(bgCtx, subscriber.ID); err != nil {
+				fmt.Printf("Failed to provision eSIM profile for subscriber %d: %v\n", subscriber.ID, err)
+			}
+		}()
+	} else {
+		subscriber.Status = models.SubscriberStatusActive
+		subscriber.ProfileStatus = models.ProfileStatusActive
+		s.db.UpdateSubscriber(ctx, subscriber)
+	}
+
+	return subscriber, nil
+}
+
+// GetSubscriber retrieves a subscriber by ID.
+func (s *SubscriberService) GetSubscriber(ctx context.Context, id uint) (*models.Subscriber, error) {
+	subscriber, err := s.db.GetSubscriber(ctx, id)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get subscriber: %w", err)
+	}
+	return subscriber, nil
+}
+
+// GetSubscriberByIMSI retrieves a subscriber by IMSI.
+func (s *SubscriberService) GetSubscriberByIMSI(ctx context.Context, imsi models.IMSI) (*models.Subscriber, error) {
+	subscriber, err := s.db.GetSubscriberByIMSI(ctx, imsi)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get subscriber by IMSI: %w", err)
+	}
+	return subscriber, nil
+}
+
+// UpdateSubscriber updates subscriber information.
+func (s *SubscriberService) UpdateSubscriber(ctx context.Context, id uint, req *UpdateSubscriberRequest) (*models.Subscriber, error) {
+	subscriber, err := s.db.GetSubscriber(ctx, id)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get subscriber: %w", err)
+	}
+
+	if req.FirstName != nil && *req.FirstName != "" {
+		subscriber.FirstName = *req.FirstName
+	}
+	if req.LastName != nil && *req.LastName != "" {
+		subscriber.LastName = *req.LastName
+	}
+	if req.Email != nil && *req.Email != "" {
+		subscriber.Email = *req.Email
+	}
+	if req.Status != "" {
+		subscriber.Status = req.Status
+	}
+	if req.PlanID != nil {
+		subscriber.PlanID = *req.PlanID
+	}
+
+	if err := s.db.UpdateSubscriber(ctx, subscriber); err != nil {
+		return nil, fmt.Errorf("failed to update subscriber: %w", err)
+	}
+
+	return subscriber, nil
+}
+
+// ListSubscribers lists subscribers with pagination and filtering.
+func (s *SubscriberService) ListSubscribers(ctx context.Context, req *ListSubscribersRequest) (*ListSubscribersResponse, error) {
+	dbReq := &database.ListSubscribersRequest{
+		Page:           req.Page,
+		PageSize:       req.PageSize,
+		Status:         req.Status,
+		OrganizationID: req.OrganizationID,
+		Search:         req.Search,
+	}
+	subscribers, total, err := s.db.ListSubscribers(ctx, dbReq)
+	if err != nil {
+		return nil, fmt.Errorf("failed to list subscribers: %w", err)
+	}
+
+	return &ListSubscribersResponse{
+		Subscribers: subscribers,
+		Total:       total,
+		Page:        req.Page,
+		PageSize:    req.PageSize,
+	}, nil
+}