feat(backend): allow regular users and tenant admins to list their full tenant trees

Changes the /v1/admin/tenants endpoint to be accessible by all authenticated users (requireAnyUser). In the handler, it dynamically resolves the user's affiliations and filters the response to return the complete hierarchical tree (root, parent, child, sibling nodes) for any tenant they belong to.

backend/cmd/server/main.go

@@ -609,10 +609,11 @@ func main() {
 	        KetoService:  ketoService,
 	})
 
-	admin.Get("/check", adminHandler.CheckAuth) // 기본 Admin 체크는 requireAdmin 없이 ApiKeyAuth로만 보호될 수 있음 (또는 추가 가능)	admin.Get("/stats", requireSuperAdmin, adminHandler.GetSystemStats)
+	admin.Get("/check", adminHandler.CheckAuth) // 기본 Admin 체크는 requireAdmin 없이 ApiKeyAuth로만 보호될 수 있음 (또는 추가 가능)
+	admin.Get("/stats", requireSuperAdmin, adminHandler.GetSystemStats)
 
 	// Tenant Management (Mixed roles, handler filters results)
-	admin.Get("/tenants", requireAdmin, tenantHandler.ListTenants)
+	admin.Get("/tenants", requireAnyUser, tenantHandler.ListTenants)
 	admin.Post("/tenants", requireSuperAdmin, tenantHandler.CreateTenant)
 	admin.Delete("/tenants/bulk", requireSuperAdmin, tenantHandler.DeleteTenantsBulk)
 	admin.Post("/tenants/:id/approve", requireSuperAdmin, tenantHandler.ApproveTenant)