docs(spec): cover WebDAV custom domain behavior

spec/system.feature

@@ -2,6 +2,12 @@ Feature: System options
   Admins manage instance-wide system options (public and private), default quota
   values, captcha, and read instance info + the release changelog.
 
+  @system/webdav-url @api
+  Scenario: The effective WebDAV URL is exposed as a read-only option
+    Given an optional WebDAV public hostname
+    When system options are listed or the WebDAV option is requested
+    Then the effective URL is public and cannot be mutated through the options API
+
   @system/option-not-found @api
   Scenario: An unknown option key returns 404
     Given no option for a key

spec/webdav.feature

@@ -21,6 +21,12 @@ Feature: WebDAV
     When PROPFIND is issued
     Then the mount root, workspace root, and folder children are listed
 
+  @webdav/custom-host @api
+  Scenario: A dedicated hostname exposes WebDAV at its root
+    Given a configured WebDAV public hostname
+    When DAV methods address resources through that hostname
+    Then response hrefs and destinations are root-relative while the main site keeps the /dav mount
+
   @webdav/propfind-workspaces @api
   Scenario: PROPFIND hides non-member workspaces
     Given several workspaces