feat(api): add realtime stats endpoints for masters

Introduce StatsService integration to admin and master handlers,
exposing realtime metrics (requests, tokens, QPS, rate limit status)
via new endpoints:
- GET /admin/masters/:id/realtime
- GET /v1/realtime

Also embed realtime stats in the existing GET /admin/masters/:id
response and change GlobalQPS default to 0 with validation to
reject negative values.

internal/api/admin_issue_key_test.go

@@ -34,7 +34,8 @@ func TestAdmin_IssueChildKeyForMaster_IssuedByAdminAndSynced(t *testing.T) {
 
 	syncService := service.NewSyncService(rdb)
 	masterService := service.NewMasterService(db)
-	adminHandler := NewAdminHandler(db, db, masterService, syncService, nil)
+	statsService := service.NewStatsService(rdb)
+	adminHandler := NewAdminHandler(db, db, masterService, syncService, statsService, nil)
 
 	m, _, err := masterService.CreateMaster("m1", "default", 5, 10)
 	if err != nil {