Provide endpoints to submit tasks to re-generate previews

The scaling factor can be given in the config file. When this changes,
images can be regenerated via POSTing to certain endpoints. It is
possible to regenerate just one attachment preview or all within a
collective.

modules/joex/src/main/resources/reference.conf

@@ -172,6 +172,18 @@ docspell.joex {
       min-text-len = 500
     }
 
+    preview {
+      # When rendering a pdf page, use this dpi. This results in
+      # scaling the image. A standard A4 page rendered at 96dpi
+      # results in roughly 790x1100px image. Using 32 results in
+      # roughly 200x300px image.
+      #
+      # Note, when this is changed, you might want to re-generate
+      # preview images. Check the api for this, there is an endpoint
+      # to regenerate all for a collective.
+      dpi = 32
+    }
+
     # Extracting text using OCR works for image and pdf files. It will
     # first run ghostscript to create a gray image from a pdf. Then
     # unpaper is run to optimize the image for the upcoming ocr, which