feat!: drop the built-in auth in favour of Authelia

BREAKING CHANGE: PASSWORD is gone and AUTH and CERTRESOLVER are required.
The server's compose.yaml and .env must be updated in the same deploy — the
new image ignores PASSWORD, and the old one refuses to start without it.

Authelia now sits in front of Traefik, so the app was asking for a second
password at the same door. Two prompts, and the weaker of the two was the one
holding a single shared secret with no sessions, no MFA and no revocation.
Deleting it is the whole change: Authelia already does this properly, once,
for every service on the host.

Gone: auth(), challenge(), the whole of throttle.go and its tests, and
golang.org/x/time with them. routes() returns the bare mux, /healthz is an
ordinary route on it, and the smoke script drops sixty -u flags. Roughly 230
lines removed and nothing written to replace them.

What holds the app up now, both asserted in compose.yaml:

- The router names the Authelia middleware through AUTH. Traefik takes a
  router out of service when its middleware does not resolve, so a typo or an
  unset variable fails shut rather than serving the app open.
- The container still publishes no ports, so the proxy is the only thing that
  can reach it. Publishing 8080 would now bypass authentication outright, not
  merely TLS — the comment there says so.

certresolver replaces the bare tls=true, parameterised as CERTRESOLVER: the
server had been carrying that label by hand since the first deploy. Naming a
resolver implies tls=true, so it stays one label.

TestAuth and TestHealthzSkipsAuth are replaced by one test asserting every
route answers without credentials — a 401 from here would now mean auth had
crept back in.

.env.example

@@ -5,13 +5,18 @@
 REPO=registry.example.com/you/foodster
 TAG=latest
 
-# Shared household password. The app will not start without it.
-PASSWORD=changeme
-
 # Hostname Traefik routes to. Kept here rather than in compose.yaml so no
 # infrastructure detail is committed.
 HOST=foodster.example.com
 
+# The Traefik middleware that authenticates the app. The app itself has no
+# login, so this is the whole of its access control — an unset or misspelt
+# name takes the router out of service, which is the right way to fail.
+AUTH=authelia@docker
+
+# Traefik certificate resolver issuing the TLS certificate for HOST.
+CERTRESOLVER=letsencrypt
+
 # Anything other than prod is written into the browser tab title, so a dev
 # instance open beside the real one can be told apart.
 ENV=prod