Use a minimum age of items to remove

In order to keep deleted items for a while, the periodic task can now
use a duration to only remove items with a certain age. This can be
used to ensure that a deleted item stays at least X days before it
will be removed from the database.

Refs: #347

build.sbt

@@ -242,6 +242,10 @@ val openapiScalaSettings = Seq(
         field =>
           field
             .copy(typeDef = TypeDef("SearchMode", Imports("docspell.common.SearchMode")))
+      case "duration" =>
+        field =>
+          field
+            .copy(typeDef = TypeDef("Duration", Imports("docspell.common.Duration")))
     }))
 )