refactor(#3393): stop restarting authelia from swarmctl

authelia now watches the users file, so the restart is redundant -- and it
was the wrong shape twice over. It could fail: a login was refused for a
user whose record was already correct on disk, with nothing in either log
implicating the reload. And it only ever worked for this writer --
swarm-authelia-bridge writes the same file and cannot restart anything,
since running unprivileged inside the container is the whole reason it may
write it at all. A reload that depends on which process did the writing is
not a reload.

--machine/--unit and their two env vars existed solely to name a
systemctl -M target, so they go with it. That drops two required settings
from the operator surface.

The three objections previously recorded against watch are all answered
now, and are kept next to the decision rather than deleted: the key is
verified against the pinned build (validate-config accepts it and rejects
a misspelling), the watch is on the directory so a rename is observed, and
partial reads are structurally impossible because every writer of this
file goes through write_atomic.

nix/host-modules/swarm-controller.nix

@@ -29,8 +29,9 @@ let
     # parameters baked into a hash have to match the verifier's.
     SWARMCTL_AUTHELIA_BIN = "${autheliaCfg.package}/bin/authelia";
     SWARMCTL_AUTHELIA_USERS_FILE = autheliaCfg.hostUsersFile;
-    SWARMCTL_AUTHELIA_MACHINE = autheliaCfg.machine;
-    SWARMCTL_AUTHELIA_UNIT = autheliaCfg.unit;
+    # No MACHINE/UNIT here any more: `swarmctl` no longer restarts authelia,
+    # because authelia watches the users file itself. Those two values existed
+    # solely to name a `systemctl -M` target.
   };
 
   natsCfg = config.services.hyperhive.swarm.nats;