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_subscriptions.go

@@ -0,0 +1,45 @@
+package graphql
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+)
+
+// SubscriberUpdates streams subscriber updates via a channel.
+func (r *Resolver) SubscriberUpdates(ctx context.Context, subscriberId string) (<-chan *models.Subscriber, error) {
+	return r.subscriber.SubscribeToSubscriberUpdates(ctx, subscriberId)
+}
+
+// UsageUpdates streams usage-event updates for an IMSI.
+func (r *Resolver) UsageUpdates(ctx context.Context, imsi string) (<-chan *models.UsageEvent, error) {
+	return r.charging.SubscribeToUsageUpdates(ctx, imsi)
+}
+
+// AlertUpdates streams alert updates filtered by severity.
+func (r *Resolver) AlertUpdates(ctx context.Context, severity models.AlertSeverity) (<-chan *models.Alert, error) {
+	severityStr := string(severity)
+	return r.subscriber.SubscribeToAlertUpdates(ctx, &severityStr)
+}
+
+// SystemStatsUpdates streams system stats updates.
+func (r *Resolver) SystemStatsUpdates(ctx context.Context) (<-chan *models.SystemStats, error) {
+	return r.charging.SubscribeToSystemStatsUpdates(ctx)
+}
+
+// ChargingSessions resolves a paginated connection of charging sessions.
+// Placeholder implementation until the charging query surface is finalized.
+func (r *Resolver) ChargingSessions(ctx context.Context, first *int, after *string, imsi *string, status *SessionStatus) (*ChargingSessionConnection, error) {
+	return &ChargingSessionConnection{
+		Edges:      []*ChargingSessionEdge{},
+		PageInfo:   &PageInfo{},
+		TotalCount: 0,
+	}, nil
+}
+
+// ChargingSession resolves a single charging session by ID.
+// Placeholder implementation until the charging query surface is finalized.
+func (r *Resolver) ChargingSession(ctx context.Context, sessionId string) (*ChargingSession, error) {
+	return nil, fmt.Errorf("charging session %s not found", sessionId)
+}