Custom domains for public release pages

Currently, public release pages are reachable under the `public.axodotdev.host` domain, where the path is something like `/releases/github/:user/:project/:package`.

We want to support being able to set custom domains for these pages, so that URLs look like this:

`my.custom.domain.org/:project/:package`

For now, we will scope these to being per user/org.

No files, tests, or entry points are identified in the issue. Start by reviewing how public release pages are currently routed under public.axodotdev.host, then define the per-user or per-organization custom-domain behavior and verify that URLs follow the requested /:project/:package shape.

Written by the indexing model from the issue text.