appview/oauth/handler: fix client metadata json file path

According to this patch: https://github.com/bluesky-social/atproto/blob/5f37211cf550b31a693cb174f1fe1d0cfee78294/packages/oauth/oauth-provider-ui/CHANGELOG.md?plain=1#L143 the client metadata json files must be served exactly at <hostname>/oauth-client-metadata.json in order to not have the PDS authentication page display an ugly json url. The file handler is also introduced on the main router because of the wildcard user handle routes. Signed-off-by: edouardparis <m@edouard.paris>