limit study chapter nodes to 3000

I've seen some with 35,000 nodes on prod.
It works but it's slow. And pointless.

modules/study/src/main/Chapter.scala

@@ -79,10 +79,16 @@ case class Chapter(
   def withoutChildren = copy(root = root.withoutChildren)
 
   def relayAndTags = relay map { Chapter.RelayAndTags(id, _, tags) }
+
+  def isOverweight = root.children.countRecursive >= Chapter.maxNodes
 }
 
 object Chapter {
 
+  // I've seen chapters with 35,000 nodes on prod.
+  // It works but could be used for DoS.
+  val maxNodes = 3000
+
   case class Id(value: String) extends AnyVal with StringValue
   implicit val idIso = lila.common.Iso.string[Id](Id.apply, _.value)
 

modules/study/src/main/ExplorerGame.scala

@@ -21,7 +21,10 @@ private final class ExplorerGame(
     }
 
   def insert(userId: User.ID, study: Study, position: Position, gameId: Game.ID): Fu[Option[(Chapter, Path)]] =
-    importer(gameId) map {
+    if (position.chapter.isOverweight) {
+      logger.info(s"Overweight chapter ${study.id}/${position.chapter.id}")
+      fuccess(none)
+    } else importer(gameId) map {
       _ ?? { game =>
         position.node ?? { fromNode =>
           GameToRoot(game, none, false).|> { root =>

modules/study/src/main/Node.scala

@@ -198,6 +198,10 @@ object Node {
           case (node, index) => node.children.pathToIndexes(tail).map(rest => index :: rest)
         }
       }
+
+    def countRecursive: Int = nodes.foldLeft(nodes.size) {
+      case (count, n) => count + n.children.countRecursive
+    }
   }
   val emptyChildren = Children(Vector.empty)
 

modules/study/src/main/StudyApi.scala

@@ -192,12 +192,16 @@ final class StudyApi(
       }
     }
 
-  private[study] def doAddNode(userId: User.ID, study: Study, position: Position, rawNode: Node, uid: Uid, opts: MoveOpts, relay: Option[Chapter.Relay]): Funit = {
+  private def doAddNode(userId: User.ID, study: Study, position: Position, rawNode: Node, uid: Uid, opts: MoveOpts, relay: Option[Chapter.Relay]): Funit = {
     val node = rawNode.withoutChildren
-    position.chapter.addNode(node, position.path, relay) match {
+    def failReload = reloadUidBecauseOf(study, uid, position.chapter.id)
+    if (position.chapter.isOverweight) {
+      logger.info(s"Overweight chapter ${study.id}/${position.chapter.id}")
+      fuccess(failReload)
+    } else position.chapter.addNode(node, position.path, relay) match {
       case None =>
-        fufail(s"Invalid addNode ${study.id} ${position.ref} $node") >>-
-          reloadUidBecauseOf(study, uid, position.chapter.id) inject none
+        failReload
+        fufail(s"Invalid addNode ${study.id} ${position.ref} $node")
       case Some(chapter) =>
         chapter.root.nodeAt(position.path) ?? { parent =>
           val newPosition = position.ref + node