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>