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/restapi/src/main/resources/docspell-openapi.yml

@@ -2526,6 +2526,24 @@ paths:
               schema:
                 type: string
                 format: binary
+    post:
+      tags: [ Attachment ]
+      summary: (Re)generate a preview image.
+      description: |
+        Submits a task that generates a preview image for this
+        attachment. The existing preview will be replaced.
+      security:
+        - authTokenHeader: []
+      parameters:
+        - $ref: "#/components/parameters/id"
+      responses:
+        200:
+          description: Ok
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/BasicResult"
+
   /sec/attachment/{id}/meta:
     get:
       tags: [ Attachment ]