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/swarm.nix

@@ -91,7 +91,7 @@ let
   # unrepresentable rather than merely detected: a default set is all or
   # nothing, and the assertion is then only ever about what an operator
   # typed.
-  queueLocal = swarmCfg.nats.enable && swarmCfg.authelia.enable && cfg.hiveName != null;
+  queueLocal = swarmCfg.nats.enable && deployCfg.authelia && cfg.hiveName != null;
 in
 {
   options.services.hyperhive.swarm.hives = lib.mkOption {