[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
Every hive with hyperhive enabled ran its own hive-forge container, and
its gateway answered forge.<swarm> with its own bridge IP, so on a
multi-host swarm each hive talked to its own forge.
deploy.forgejo.enable defaults to false and allSwarmServices sets it with
mkDefault, like authelia and bao; singleHostSwarm gets it through that.
The forge's OIDC client moves to a glue module gated on authelia, so a
split authelia/forge swarm still registers it. CI now requires the forge
on the same host, and the controller's forgeTokenFile defaults to null
where the forge is not.
Closes#4705
Refs #3782
