Fix storing empty-trash task

It was wrongly stored using RPeriodicTask directly, but the higher
level `UserTask` must be used instead, because this ensures a
correctly scoped periodic task using the `updateOneTask` method. Since
this is a system task, it can be given a fixed ID which makes it now
safe even if stored using RPeriodicTask directly.

The bug resulted in multiple empty-trash tasks to be inserted (on each
restart).

Refs: #347

modules/common/src/main/scala/docspell/common/EmptyTrashArgs.scala

@@ -25,6 +25,8 @@ case class EmptyTrashArgs(
   def makeSubject: String =
     s"Empty Trash: Remove older than ${minAge.toJava}"
 
+  def periodicTaskId: Ident =
+    EmptyTrashArgs.periodicTaskId(collective)
 }
 
 object EmptyTrashArgs {
@@ -33,6 +35,9 @@ object EmptyTrashArgs {
 
   val defaultSchedule = CalEvent.unsafe("*-*-1/7 03:00:00")
 
+  def periodicTaskId(coll: Ident): Ident =
+    Ident.unsafe(s"docspell") / taskName / coll
+
   implicit val jsonEncoder: Encoder[EmptyTrashArgs] =
     deriveEncoder[EmptyTrashArgs]
   implicit val jsonDecoder: Decoder[EmptyTrashArgs] =