Replace generating preview images with an admin endpoint

It doesn't make much sense to have this per collective, because this
is triggered by an admin after changing the server config file. So it
is now implemented as an admin endpoint that affects all files.

modules/backend/src/main/scala/docspell/backend/ops/OItem.scala

@@ -193,6 +193,14 @@ trait OItem[F[_]] {
       account: AccountId,
       notifyJoex: Boolean
   ): F[UpdateResult]
+
+  /** Submits a task that (re)generates the preview images for all
+    * attachments.
+    */
+  def generateAllPreviews(
+      storeMode: MakePreviewArgs.StoreMode,
+      notifyJoex: Boolean
+  ): F[UpdateResult]
 }
 
 object OItem {
@@ -699,6 +707,16 @@ object OItem {
             _   <- if (notifyJoex) joex.notifyAllNodes else ().pure[F]
           } yield UpdateResult.success
 
+        def generateAllPreviews(
+            storeMode: MakePreviewArgs.StoreMode,
+            notifyJoex: Boolean
+        ): F[UpdateResult] =
+          for {
+            job <- JobFactory.allPreviews[F](AllPreviewsArgs(None, storeMode), None)
+            _   <- queue.insertIfNew(job)
+            _   <- if (notifyJoex) joex.notifyAllNodes else ().pure[F]
+          } yield UpdateResult.success
+
         private def onSuccessIgnoreError(update: F[Unit])(ar: UpdateResult): F[Unit] =
           ar match {
             case UpdateResult.Success =>