Isolate internal bot-config route from the public listener (#33)

The GET /internal/bot-config route returns the DECRYPTED Discord bot
token and was mounted on the same Express app / port 3000 that Pangolin
proxies publicly. Its only guard was the BOT_INTERNAL_KEY shared secret,
and .env.example shipped a placeholder default — so a forwarded path or a
weak/unrotated key would expose the plaintext token to the internet.

Move server<->bot internal traffic onto its own listener and fail fast on
a weak key:

- Add server/src/internalApp.js: a standalone Express app mounting
  requireInternalKey + /internal (and a no-secret /health), mirroring the
  bot's unpublished port-4100 pattern.
- server.js starts a second listener on INTERNAL_PORT (default 3001),
  closed on graceful shutdown.
- Remove the /internal mount from the public v1.router; the public app now
  404s /api/v1/internal/bot-config even with a valid key.
- Fail fast: new utils/botInternalKey.js rejects an empty, placeholder, or
  <16-char BOT_INTERNAL_KEY — fatal in production (exit 1), warning in dev.
- docker-compose: bot SITE_INTERNAL_URL -> app:3001/internal/bot-config;
  document that INTERNAL_PORT stays unpublished.
- .env.example (root/server/bot): document INTERNAL_PORT, the fail-fast
  behavior, and a defense-in-depth Pangolin deny rule for /api/v1/internal.

Tests: add requireInternalKey.test.js and botInternalKey.test.js
(node --test: 93 pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019rao86n5cXpwAyjdBFEshV

server/src/internalApp.js

@@ -0,0 +1,26 @@
+// Standalone Express app for server<->bot internal traffic. It is mounted on its
+// OWN http listener (INTERNAL_PORT, default 3001) in server.js — an unpublished,
+// compose-network-only port, mirroring how the bot exposes its internal API on
+// 4100. Crucially it is NOT part of the public API app (app.js), so /internal/*
+// (which returns the DECRYPTED Discord bot token) can never ride the same
+// listener Pangolin proxies to the world. Shared-secret gated by
+// requireInternalKey inside internal.routes. See issue #33.
+const express = require('express')
+
+const internalRouter = require('./router/v1/internal/internal.routes')
+
+const internalApp = express()
+
+internalApp.use(express.json())
+
+// Liveness probe for this listener (no secret required); mirrors the bot's
+// /health on 4100. Useful for compose healthchecks without exposing anything.
+internalApp.get('/health', (req, res) => res.json({ status: 'ok' }))
+
+// requireInternalKey is applied inside internal.routes.
+internalApp.use('/internal', internalRouter)
+
+// Anything else on this listener is not a real internal route.
+internalApp.use((req, res) => res.status(404).json({ message: 'Not found' }))
+
+module.exports = internalApp