matrix-media-repo is not OIDC aware, and so cannot check if the requestor for admin operations is actually an admin

[Half-Shot]

https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html#get-whether-a-user-is-a-server-administrator-or-not is disabled when MSC3861 is enabled, so it's impossible to make any admin requests when using a token minted by an OIDC provider like matrix-authentication-service.

https://github.com/t2bot/matrix-media-repo/blob/main/matrix/requests_admin.go#L20-L26

I suspect the proper fix here is to make MMR OIDC aware and properly introspect the token.