[oauth2_client] turns on auto-registration through the authelia login
source, with the account named after authelia's preferred_username.
DISABLE_REGISTRATION stays true: forgejo 16's auto-registration checks
only ALLOW_ONLY_INTERNAL_REGISTRATION, so local sign-up stays off.
ACCOUNT_LINKING is `login`, forgejo's default, set explicitly. With
`auto`, an SSO login whose name matches an existing local account would
be handed that account, and agents, `core` and `swarm-controller` all
have one. `login` asks for that account's own password instead.
Refs #3782
113f3fe6e2