Custom domains for artifact hosting

Artifacts hosted on axo Releases are currently reachable at `.artifacts.axodotdev.host/:package/:tag/:filename`. We want to be able to support custom domains for this, too, so that those artifacts would be reachable at `my.custom.domain.org/:package/:tag/:filename`.

Start by tracing how axo Releases currently routes and serves artifact URLs, since the issue names no files, tests, or entry points. Define the custom-domain configuration, routing, and validation requirements; the work is done when artifacts are reachable through a configured custom domain using the stated package, tag, and filename path.

Written by the indexing model from the issue text.