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/store/src/main/resources/db/migration/h2/V1.25.3__delete_corrupt_trash_task.sql

@@ -0,0 +1,3 @@
+-- note this is only for users of nightly releases
+DELETE FROM "periodic_task"
+WHERE "task" = 'empty-trash';

modules/store/src/main/resources/db/migration/mariadb/V1.25.3__delete_corrupt_trash_task.sql

@@ -0,0 +1,3 @@
+-- note this is only for users of nightly releases
+DELETE FROM `periodic_task`
+WHERE `task` = 'empty-trash';

modules/store/src/main/resources/db/migration/postgresql/V1.25.3__delete_corrupt_trash_task.sql

@@ -0,0 +1,3 @@
+-- note this is only for users of nightly releases
+DELETE FROM "periodic_task"
+WHERE "task" = 'empty-trash';

modules/store/src/main/scala/docspell/store/usertask/UserTaskStore.scala

@@ -81,7 +81,8 @@ trait UserTaskStore[F[_]] {
     *
     * Unlike `updateTask`, this ensures that there is at most one task of some name in the
     * db. Multiple same tasks (task with same name) may not be allowed to run, depending
-    * on what they do. This is not ensured by the database, though.
+    * on what they do. This is not ensured by the database, though. The task is identified
+    * by task name, submitter and group.
     *
     * If there are currently multiple tasks with same name as `ut` for the user `account`,
     * they will all be removed and the given task inserted!