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.

go.sum

@@ -50,8 +50,6 @@ golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
 golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
-golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
 golang.org/x/tools v0.48.0 h1:3+hClM1aLL5mjMKm5ovokw9epgRXPuu2tILgismM6RE=
 golang.org/x/tools v0.48.0/go.mod h1:08xX0orndb/F7jJxGDicx061tyd5pcMto75YMAXr6lk=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=