The prefix was only trimmed. Without a leading slash ServeMux read its first
segment as a host pattern, so every upload silently answered 404, and a CDN
URL panicked in ServeMux and surfaced as a misleading route conflict. With
storage.uploads.serve on, BuildRouter now refuses a prefix that is not an
absolute URL path (scheme, host, query, fragment, whitespace, wildcard
braces, empty, . or .. segments, or a bare /) with an error naming the key.