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_subscriptions.go

@@ -0,0 +1,96 @@
+package services
+
+import (
+	"context"
+	"strconv"
+	"time"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// SubscribeToSubscriberUpdates returns a channel that emits the subscriber whenever
+// their row is updated. Implemented via polling; swap for LISTEN/NOTIFY when available.
+func (s *SubscriberService) SubscribeToSubscriberUpdates(ctx context.Context, subscriberId string) (<-chan *models.Subscriber, error) {
+	ch := make(chan *models.Subscriber, 5)
+
+	go func() {
+		defer close(ch)
+
+		ticker := time.NewTicker(3 * time.Second)
+		defer ticker.Stop()
+
+		var lastUpdatedAt time.Time
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				var subscriber models.Subscriber
+				query := s.db.DB.WithContext(ctx).Where("imsi = ?", subscriberId)
+				if !lastUpdatedAt.IsZero() {
+					query = query.Where("updated_at > ?", lastUpdatedAt)
+				}
+
+				if err := query.First(&subscriber).Error; err != nil {
+					continue
+				}
+
+				select {
+				case ch <- &subscriber:
+					lastUpdatedAt = subscriber.UpdatedAt
+				case <-ctx.Done():
+					return
+				}
+			}
+		}
+	}()
+
+	return ch, nil
+}
+
+// SubscribeToAlertUpdates returns a channel that emits newly created alerts, optionally
+// filtered by subscriberId (numeric string).
+func (s *SubscriberService) SubscribeToAlertUpdates(ctx context.Context, subscriberId *string) (<-chan *models.Alert, error) {
+	ch := make(chan *models.Alert, 10)
+
+	go func() {
+		defer close(ch)
+
+		ticker := time.NewTicker(2 * time.Second)
+		defer ticker.Stop()
+
+		var lastAlertID uint
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				var alerts []models.Alert
+				query := s.db.DB.WithContext(ctx).Where("id > ?", lastAlertID)
+
+				if subscriberId != nil {
+					if subID, err := strconv.ParseUint(*subscriberId, 10, 32); err == nil {
+						query = query.Where("subscriber_id = ?", subID)
+					}
+				}
+
+				if err := query.Order("id ASC").Limit(10).Find(&alerts).Error; err != nil {
+					continue
+				}
+
+				for _, alert := range alerts {
+					select {
+					case ch <- &alert:
+						lastAlertID = alert.ID
+					case <-ctx.Done():
+						return
+					}
+				}
+			}
+		}
+	}()
+
+	return ch, nil
+}