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>

Dockerfile

@@ -50,6 +50,14 @@ ENV HOSTNAME="0.0.0.0"
 #                     bundle. Generate with: openssl rand -base64 48
 #                     → NOT shipped. Set it as a Dokploy secret.
 #
+# A container started WITHOUT AUTH_SECRET no longer dies. It boots, prints the
+# missing variable on stderr, answers 503 with `x-loyaly-config: misconfigured`
+# on every request, and fails the HEALTHCHECK below. Exiting instead is what
+# made this fault present as a bare 502 Bad Gateway from the reverse proxy on
+# every url — /favicon.ico first, in the browser console — with the one line
+# that explained it trapped inside a restart-looping container. See
+# src/instrumentation-node.ts.
+#
 # AUTH_SECRET used to be an ENV line here with a literal value, which put a
 # session-forging key in git: anyone who could read the repo could mint a
 # cookie for any user, and every built image carried it in a layer that
@@ -90,4 +98,23 @@ USER nextjs
 
 EXPOSE 3000
 
+# Is this container able to serve, or merely running?
+#
+# /api/health answers 200 only when every required variable is present and
+# acceptable, and 503 otherwise, so a container missing AUTH_SECRET reports
+# `unhealthy` rather than presenting itself as a working deploy. That is the
+# property the old process.exit(1) was protecting; this keeps it without
+# taking the site down to do it.
+#
+# Probed with node rather than curl or busybox wget: node is already the
+# container's entrypoint, so this adds no package and cannot break because a
+# base image dropped an applet. `r.ok` is the check — a 503 from /api/health
+# exits 1, which is what marks the container unhealthy — and the catch covers
+# a server that is not listening at all.
+#
+# start-period covers first boot: Next reports ready in well under 20s here,
+# and an unready server must not be reported as a broken one.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD node -e "fetch('http://127.0.0.1:'+(process.env.PORT||3000)+'/api/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+
 CMD ["node", "server.js"]