feat: Refactor GraphQL resolvers to extract domain methods into separate files and retain only shared helpers

- Add package documentation mapping resolver files to domains (subscriber/usage/billing/mutations/esim/alerts/subscriptions)
- Remove models package import and add blank imports for context/base64 to preserve helper function dependencies
- Remove all resolver method implementations (Subscriber/SubscriberByImsi/Subscribers/SubscriberAccount/UsageEvents/UsageStats/RealTimeUsage/Invoices/Invoice

apps/api-server/internal/graphql/resolvers_esim.go

@@ -0,0 +1,59 @@
+package graphql
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// ProvisionESIM provisions an eSIM profile for the given IMSI.
+func (r *Resolver) ProvisionESIM(ctx context.Context, imsi string) (*ESIMProvisionResult, error) {
+	sub, err := r.subscriber.GetSubscriberByIMSI(ctx, models.IMSI(imsi))
+	if err != nil {
+		return nil, fmt.Errorf("subscriber not found for IMSI %s: %w", imsi, err)
+	}
+	result, err := r.es2Service.ProvisionProfile(ctx, sub)
+	if err != nil {
+		return nil, err
+	}
+	return &ESIMProvisionResult{
+		ProfileID:      result.ProfileID,
+		ActivationCode: result.Activation.ActivationCode,
+	}, nil
+}
+
+// ActivateESIM activates the given eSIM profile.
+func (r *Resolver) ActivateESIM(ctx context.Context, imsi string, profileId string) (bool, error) {
+	err := r.es2Service.ActivateProfile(ctx, imsi, profileId)
+	return err == nil, err
+}
+
+// DeactivateESIM deactivates the given eSIM profile.
+func (r *Resolver) DeactivateESIM(ctx context.Context, imsi string, profileId string) (bool, error) {
+	err := r.es2Service.DeactivateProfile(ctx, imsi, profileId)
+	return err == nil, err
+}
+
+// AddPaymentMethod adds a payment method for the given subscriber.
+func (r *Resolver) AddPaymentMethod(ctx context.Context, subscriberId int, input PaymentMethodInput) (*models.PaymentMethod, error) {
+	isDefault := false
+	if input.IsDefault != nil {
+		isDefault = *input.IsDefault
+	}
+	return r.subscriber.AddPaymentMethod(ctx, subscriberId, &models.AddPaymentMethodRequest{
+		Type:      models.PaymentMethodType(input.Type),
+		Token:     input.Token,
+		IsDefault: isDefault,
+	})
+}
+
+// RemovePaymentMethod removes a payment method.
+func (r *Resolver) RemovePaymentMethod(ctx context.Context, paymentMethodId string) (bool, error) {
+	return r.subscriber.RemovePaymentMethod(ctx, paymentMethodId)
+}
+
+// SetDefaultPaymentMethod marks a payment method as default.
+func (r *Resolver) SetDefaultPaymentMethod(ctx context.Context, paymentMethodId string) (*models.PaymentMethod, error) {
+	return r.subscriber.SetDefaultPaymentMethod(ctx, paymentMethodId)
+}