Allow to configure a region for s3 backend

Closes: #2386

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

@@ -493,6 +493,7 @@ docspell.server {
            access-key = "username"
            secret-key = "password"
            bucket = "docspell"
+           region = ""
          }
       }
     }

modules/restserver/src/main/scala/docspell/restserver/ConfigFile.scala

@@ -113,7 +113,9 @@ object ConfigFile {
     }
 
   def filesValidate: Validation[Config] =
-    Validation(cfg => cfg.backend.files.validate.map(_ => cfg))
+    Validation(cfg =>
+      cfg.backend.files.validate.map(f => cfg.copy(backend = cfg.backend.copy(files = f)))
+    )
 
   def postgresFtsValidate: Validation[Config] =
     Validation.failWhen(