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.

Makefile

@@ -4,7 +4,7 @@ COMPOSE ?= podman compose
 BIN     := foodster
 PKG     := ./cmd/foodster
 
-# Shared password and TZ live here. Gitignored.
+# Registry coordinates, hostname, TZ. Gitignored.
 ifneq (,$(wildcard .env))
 include .env
 export
@@ -28,7 +28,7 @@ build: generate ## Build ./foodster
 		-ldflags="-s -w -X main.version=dev" -o $(BIN) $(PKG)
 
 run: generate ## Run locally on :8080 (database in ./data)
-	PASSWORD=$${PASSWORD:-dev} ENV=dev go run $(PKG)
+	ENV=dev go run $(PKG)
 
 test: generate ## Run unit tests
 	go test ./...