fix(deploy): serve a 503 that names the fault instead of dying into a 502

A container started without AUTH_SECRET called process.exit(1) from the boot
check. The container died, Dokploy's Traefik had no upstream to proxy to, and
every url answered 502 Bad Gateway — /favicon.ico first, which is the line that
shows up in a browser console. The one message that explained it was on stderr
inside a restart-looping container, so the fastest fault in this app to fix
became the slowest to identify.

Reproduced against the real standalone payload: without AUTH_SECRET the process
exited 1; with it, /favicon.ico 200, /login 200, / 307.

The container now boots and stays up. While a required variable is missing the
proxy answers 503 with `x-loyaly-config: misconfigured` on every gated request,
and the boot log names the variable. A Docker HEALTHCHECK against the new
/api/health keeps the property the exit was protecting — a broken deploy still
reports unhealthy rather than presenting itself as a working one.

- configCheck.ts: one runtime-neutral check shared by the boot log, the proxy
  and the health route, memoised so a healthy server pays an array-length read
  per request rather than re-reading the environment.
- proxy.ts: the config gate runs before the session gate. verifySessionToken
  reads AUTH_SECRET and throws ConfigError without one, which Next turns into a
  500 per request — a status that says "this server has a bug" for a server that
  is merely unconfigured.
- Neither the 503 body nor /api/health names the missing variable. Those
  messages are operator information (configError.ts states the rule, the login
  route already follows it); the names go to the container log.
- HEALTHCHECK probes with node, already the entrypoint, so it adds no package
  and cannot break because a base image dropped a busybox applet.
- nginx.conf: marked dead. Nothing has installed nginx since 28258b5 and
  .dockerignore keeps it out of the build context, but it is the first place
  anyone looks at a 502 and the wrong one.

Verified: tsc --noEmit clean, eslint clean, and the Docker builder stage's
`NODE_ENV=production CI_BUILD=1 next build` exits 0. Misconfigured -> 503 on
/login, /dashboard, /api/sites with healthcheck exit 1; configured -> 200/307
with healthcheck exit 0.

This does not by itself end the outage: AUTH_SECRET still has to be set on the
container (Dokploy -> Environment). It makes the next occurrence legible.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

nginx.conf

@@ -1,3 +1,20 @@
+# ─────────────────────────────────────────────────────────────────────────────
+# NOT USED BY THE DEPLOYED IMAGE. Kept only as a reference.
+#
+# The image ran `node server.js & nginx -g 'daemon off;'` and exposed port 80
+# until commit 28258b5 ("fix docker port"), which dropped nginx and made the
+# Next standalone server the container's only process on port 3000. Nothing
+# installs nginx any more, and .dockerignore excludes this file from the build
+# context, so editing it CANNOT affect production — the TLS termination and
+# reverse proxy in front of the container are Dokploy's Traefik, configured in
+# the Dokploy dashboard, not here.
+#
+# That matters when a 502 Bad Gateway shows up: this file is the obvious place
+# to look and the wrong one. A 502 means Traefik had no healthy container to
+# proxy to. Check the container's log for `[loyaly] configuration problem` and
+# `GET /api/health` first.
+# ─────────────────────────────────────────────────────────────────────────────
+
 events {
     worker_connections 1024;
 }