deploy: move the SSO provider toggle

The largest of these moves: sixteen references spelled through `let`
aliases across eight modules, plus eight more spelled as a path, plus
five documentation pages.

authelia is also the clearest case for why the two namespaces exist.
`swarm.authelia.url` is needed by *every* hive in the swarm — it says
where to send a browser to authenticate — while running the container is
the business of exactly one host. The client half and the server half
were sharing a namespace whose whole contract is "identical everywhere",
and only one of them could honour it.

`swarm.authelia.oidc.clients` stays where it is for the same reason:
several modules register a client there, gated on authelia running here,
and the registry itself is what the service *is* rather than a decision
about this machine.

One sweep note worth recording: a grep for `swarm.authelia.enable` misses
`swarmCfg.authelia.enable`, because the prefix is whatever the reading
file bound. Grepping the suffix `.authelia.enable` finds both, and found
a reference in swarm.nix that the path-shaped pattern did not.

nix/host-modules/local-defaults.nix

@@ -95,7 +95,7 @@ in
     # ran its first boot on — so they belong to the mode that asserts this
     # box is the whole deployment, not to the options' own `default`.
     #
-    # Deriving them from `swarm.nats.enable` / `swarm.authelia.enable`
+    # Deriving them from `swarm.nats.enable` / `deploy.authelia`
     # inside those defaults is the mixing this file exists to prevent: the
     # option would be describing a deployment shape instead of describing
     # itself, and "what does all-local turn on?" would stop having one