Offline redirect URL returns 502 generic page for root but works for relative paths

Repro:

* https://hubs.mod.studio/ -> 503 Service Temporarily Unavailable
* https://hubs.mod.studio/admin -> redirects correctly to the offline page set in CF template

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1054)

Start by locating the CF template that defines the offline redirect and compare the root URL behavior with the working /admin path. Reproduce both requests and trace the root response; done means https://hubs.mod.studio/ reaches the configured offline page instead of returning a generic 503 response.

Written by the indexing model from the issue text.