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_subscriber.go

@@ -0,0 +1,73 @@
+package graphql
+
+import (
+	"context"
+	"encoding/base64"
+	"fmt"
+
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+	"github.com/nutcas3/telecom-platform/apps/api-server/internal/services"
+)
+
+// Subscriber resolves a single subscriber by GraphQL ID.
+func (r *Resolver) Subscriber(ctx context.Context, id string) (*models.Subscriber, error) {
+	sid, err := parseID(id)
+	if err != nil {
+		return nil, err
+	}
+	return r.subscriber.GetSubscriber(ctx, sid)
+}
+
+// SubscriberByImsi resolves a subscriber by IMSI.
+func (r *Resolver) SubscriberByImsi(ctx context.Context, imsi string) (*models.Subscriber, error) {
+	return r.subscriber.GetSubscriberByIMSI(ctx, models.IMSI(imsi))
+}
+
+// Subscribers resolves a paginated connection of subscribers.
+func (r *Resolver) Subscribers(ctx context.Context, first *int, after *string, filter *SubscriberFilter, sort *SubscriberSort) (*SubscriberConnection, error) {
+	limit := 20
+	if first != nil {
+		limit = *first
+	}
+	offset := 0
+	if after != nil {
+		cursor, err := base64.StdEncoding.DecodeString(*after)
+		if err != nil {
+			return nil, fmt.Errorf("invalid cursor")
+		}
+		offset = parseCursorOffset(string(cursor))
+	}
+
+	resp, err := r.subscriber.ListSubscribers(ctx, &services.ListSubscribersRequest{
+		Page:     offset/limit + 1,
+		PageSize: limit,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	edges := make([]*SubscriberEdge, len(resp.Subscribers))
+	for i, sub := range resp.Subscribers {
+		s := sub
+		edges[i] = &SubscriberEdge{
+			Node:   &s,
+			Cursor: base64.StdEncoding.EncodeToString(fmt.Appendf(nil, "%d", offset+i+1)),
+		}
+	}
+
+	return &SubscriberConnection{
+		Edges:      edges,
+		PageInfo:   buildConnectionPageInfo(offset, len(resp.Subscribers), resp.Total),
+		TotalCount: int(resp.Total),
+	}, nil
+}
+
+// SubscriberAccount resolves a subscriber's account by IMSI.
+func (r *Resolver) SubscriberAccount(ctx context.Context, imsi string) (*models.SubscriberAccount, error) {
+	return r.subscriber.GetAccount(ctx, imsi)
+}
+
+// SearchSubscribers performs a subscriber search, capped at the given limit.
+func (r *Resolver) SearchSubscribers(ctx context.Context, query string, limit int) ([]*models.Subscriber, error) {
+	return r.subscriber.SearchSubscribers(ctx, query, limit)
+}