refactor: Split subscriber service into separate files for better code organization

- Move IMSI allocation logic to subscriber_imsi.go
- Move authentication key generation (generateAuthKeys, getOperatorVariant, generateOPc) to subscriber_auth.go
- Move eSIM profile operations (provisionESIMProfile, deactivateESIMProfile) to subscriber_esim.go
- Keep core subscriber CRUD operations and session management in subscriber.go
- Remove unused crypto/aes, crypto/rand, encoding/hex, and os imports from main

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

@@ -0,0 +1,35 @@
+package services
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// allocateIMSI allocates a new IMSI for a subscriber
+func (s *SubscriberService) allocateIMSI(ctx context.Context) (models.IMSI, error) {
+	// Get current IMSI allocation
+	alloc, err := s.db.GetIMSIAllocation(ctx)
+	if err != nil {
+		return "", fmt.Errorf("failed to get IMSI allocation: %w", err)
+	}
+
+	// Check if we've reached the maximum
+	if alloc.LastIMSI >= alloc.MaxIMSI {
+		return "", fmt.Errorf("IMSI range exhausted: max %d reached", alloc.MaxIMSI)
+	}
+
+	// Allocate next IMSI
+	nextIMSI := alloc.LastIMSI + 1
+	alloc.LastIMSI = nextIMSI
+
+	// Update allocation state
+	if err := s.db.UpdateIMSIAllocation(ctx, alloc); err != nil {
+		return "", fmt.Errorf("failed to update IMSI allocation: %w", err)
+	}
+
+	// Format IMSI: MCC (3) + MNC (2-3) + subscriber number
+	imsiStr := fmt.Sprintf("%s%010d", s.config.IMSI.Prefix, nextIMSI)
+	return models.IMSI(imsiStr), nil
+}