Use source name from config file for integration endpoint uploads

Fixes: #389

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

@@ -73,6 +73,10 @@ docspell.server {
     # The priority to use when submitting files through this endpoint.
     priority = "low"
 
+    # The name used for the item "source" property when uploaded
+    # through this endpoint.
+    source-name = "integration"
+
     # IPv4 addresses to allow access. An empty list, if enabled,
     # prohibits all requests. IP addresses may be specified as simple
     # globs: a part marked as `*' matches any octet, like in