Limit annotation update transactions to 1M actions (#8866)

Allocating the string for the serialized json caused an OutOfMemoryError
for huge transactions.

1M actions is already a bit much anyway. We should rather create actions
that edit multiple things.

### Steps to test:
- Lower the limit to e.g. 10
- Brush some, should create >10 updateBucket actions
- hit save, should be rejected


### Issues:
- workaround for #8788

Author: fm3

unreleased_changes/8866.md

@@ -0,0 +1,2 @@
+### Changed
+- Added protection against server outages by limiting the number of update actions in an annotation update transaction to 1000000.

webknossos-tracingstore/app/com/scalableminds/webknossos/tracingstore/annotation/AnnotationTransactionService.scala

@@ -222,7 +222,9 @@ class AnnotationTransactionService @Inject()(handledGroupIdStore: TracingStoreRe
       implicit ec: ExecutionContext,
       tc: TokenContext): Fox[Unit] =
     for {
-      updateActionsJson <- Fox.successful(Json.toJson(preprocessActionsForStorage(updateActionGroup)))
+      updateActionsProcessed <- Fox.successful(preprocessActionsForStorage(updateActionGroup))
+      _ <- Fox.fromBool(updateActionsProcessed.length <= 1000000) ?~> "Annotation update transactions with more than 1M update actions are not currently supported"
+      updateActionsJson = Json.toJson(updateActionsProcessed)
       _ <- tracingDataStore.annotationUpdates.put(annotationId.toString, updateActionGroup.version, updateActionsJson)
       bucketMutatingActions = findBucketMutatingActions(updateActionGroup)
       actionsGrouped: Map[String, List[BucketMutatingVolumeUpdateAction]] = bucketMutatingActions.groupBy(