Create tasks to generate all previews

There is a task to generate preview images per attachment. It can
either add them (if not present yet) or overwrite them (e.g. some
config has changed).

There is a task that selects all attachments without previews and
submits a task to create it. This is submitted on start automatically
to generate previews for all existing attachments.

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

@@ -0,0 +1,26 @@
+package docspell.common
+
+import io.circe.generic.semiauto._
+import io.circe.{Decoder, Encoder}
+
+/** Arguments for the `AllPreviewsTask` that submits tasks to
+  * generates a preview image for attachments.
+  *
+  * It can replace the current preview image or only generate one, if
+  * it is missing. If no collective is specified, it considers all
+  * attachments.
+  */
+case class AllPreviewsArgs(
+    collective: Option[Ident],
+    storeMode: MakePreviewArgs.StoreMode
+)
+
+object AllPreviewsArgs {
+
+  val taskName = Ident.unsafe("all-previews")
+
+  implicit val jsonEncoder: Encoder[AllPreviewsArgs] =
+    deriveEncoder[AllPreviewsArgs]
+  implicit val jsonDecoder: Decoder[AllPreviewsArgs] =
+    deriveDecoder[AllPreviewsArgs]
+}

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

@@ -2,8 +2,8 @@ package docspell.common
 
 object DocspellSystem {
 
-  val user                 = Ident.unsafe("docspell-system")
-  val taskGroup            = user
-  val migrationTaskTracker = Ident.unsafe("full-text-index-tracker")
-
+  val user                  = Ident.unsafe("docspell-system")
+  val taskGroup             = user
+  val migrationTaskTracker  = Ident.unsafe("full-text-index-tracker")
+  val allPreviewTaskTracker = Ident.unsafe("generate-all-previews")
 }

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

@@ -0,0 +1,53 @@
+package docspell.common
+
+import io.circe.generic.semiauto._
+import io.circe.{Decoder, Encoder}
+
+/** Arguments for the `MakePreviewTask` that generates a preview image
+  * for an attachment.
+  *
+  * It can replace the current preview image or only generate one, if
+  * it is missing.
+  */
+case class MakePreviewArgs(
+    attachment: Ident,
+    store: MakePreviewArgs.StoreMode
+)
+
+object MakePreviewArgs {
+
+  val taskName = Ident.unsafe("make-preview")
+
+  sealed trait StoreMode extends Product {
+    final def name: String =
+      productPrefix.toLowerCase()
+  }
+  object StoreMode {
+
+    /** Replace any preview file that may already exist. */
+    case object Replace extends StoreMode
+
+    /** Only create a preview image, if it is missing. */
+    case object WhenMissing extends StoreMode
+
+    def fromString(str: String): Either[String, StoreMode] =
+      Option(str).map(_.trim.toLowerCase()) match {
+        case Some("replace")     => Right(Replace)
+        case Some("whenmissing") => Right(WhenMissing)
+        case _                   => Left(s"Invalid store mode: $str")
+      }
+
+    implicit val jsonEncoder: Encoder[StoreMode] =
+      Encoder.encodeString.contramap(_.name)
+
+    implicit val jsonDecoder: Decoder[StoreMode] =
+      Decoder.decodeString.emap(fromString)
+  }
+
+  implicit val jsonEncoder: Encoder[MakePreviewArgs] =
+    deriveEncoder[MakePreviewArgs]
+
+  implicit val jsonDecoder: Decoder[MakePreviewArgs] =
+    deriveDecoder[MakePreviewArgs]
+
+}