Fix attachment positions

It worked for new items, because the implicit offset was 0. when
adding archives to existing items, there are already attachments and
the new attachments are added to the end. This won't work if files are
added concurrently, because there is no quick and reliable way to
determine the offset then.

modules/store/src/main/scala/docspell/store/impl/Column.scala

@@ -101,4 +101,6 @@ case class Column(name: String, ns: String = "", alias: String = "") {
   def asc: Fragment =
     f ++ fr"asc"
 
+  def max: Fragment =
+    fr"MAX(" ++ f ++ fr")"
 }

modules/store/src/main/scala/docspell/store/records/RAttachment.scala

@@ -38,8 +38,10 @@ object RAttachment {
       fr"${v.id},${v.itemId},${v.fileId.id},${v.position},${v.created},${v.name}"
     ).update.run
 
-  def countOnItem(id: Ident): ConnectionIO[Int] =
-    selectCount(itemId, table, itemId.is(id)).query[Int].unique
+  def nextPosition(id: Ident): ConnectionIO[Int] =
+    for {
+      max <- selectSimple(position.max, table, itemId.is(id)).query[Option[Int]].unique
+    } yield max.map(_ + 1).getOrElse(0)
 
   def updateFileIdAndName(
       attachId: Ident,