fix(deploy): drop the healthcheck that reproduced the 502 it was meant to end

The HEALTHCHECK added an hour ago recreated the exact symptom. Dokploy runs
applications as Docker Swarm services, and Swarm does not merely report an
unhealthy task — it removes it from the service load balancer and reschedules
it. /api/health answers 503 while a required variable is missing, so:

  AUTH_SECRET unset -> /api/health 503 -> task unhealthy -> pulled out of the
  load balancer -> Traefik has no backend -> 502 Bad Gateway on every url.

The container was up and serving a 503 that names the fault, and nothing could
reach it to read that 503. "A broken deploy must not look healthy" is a real
concern, but enforcing it in the orchestrator destroys the diagnostics, and an
outage whose reason cannot be seen is the more expensive failure. The container
now stays in rotation whenever it can serve HTTP at all.

Also, two things that make a secret that was SET look like one that was not:

- Recommend `openssl rand -hex 32` everywhere instead of `openssl rand -base64
  48`. A base64 value ends in '=' and may contain '+' and '/'; pasted into a
  dashboard field or a KEY=VALUE editor that splits on the first '=', it can be
  stored truncated or empty, which is indistinguishable from never setting it.
  Hex is [0-9a-f] only, so there is nothing for a parser to mangle.
- Treat a whitespace-only AUTH_SECRET as missing, and print the secret's LENGTH
  (never its value) in the boot log. `openssl rand -hex 32` is 64 characters, so
  a much shorter number there is a value that arrived truncated — which
  otherwise presents as sessions that do not verify, with nothing to explain it.

Verified on the rebuilt standalone payload: absent -> container alive, 503
`x-loyaly-config: misconfigured` on /, /login and /api/sites; whitespace -> the
same; a real hex secret -> / 307, /login 200, /favicon.ico 200, /api/health 200
and `auth secret set (64 chars)` in the log. tsc --noEmit and eslint clean,
production build exits 0.

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

.env.example

@@ -29,7 +29,8 @@ LOYALY_API_BASE=http://127.0.0.1:8088
 # as a Dokploy environment variable / secret. Locally, any string works; leave
 # it blank and a development key is used.
 #
-# Generate with: openssl rand -base64 48
+# Generate with: openssl rand -hex 32  (hex, not base64 — a trailing '=' can be
+# mangled by a dashboard env editor that splits on the first '=')
 AUTH_SECRET=
 
 # Browser → this app's own BFF routes. Same origin, so leave it empty. Inlined