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  bot's unpublished port-4100 pattern.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  document that INTERNAL_PORT stays unpublished.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

docker-compose.yml

@@ -35,6 +35,10 @@ services:
       - uploads:/app/uploads
       # Bind-mount logs to the host so app.log is directly readable at ./logs/
       - ./logs:/app/logs
+    # Only the PUBLIC API port (3000) is published. The internal server<->bot
+    # port (INTERNAL_PORT, default 3001) is deliberately NOT listed here, so it
+    # stays reachable only over the private compose network — Pangolin/the public
+    # reverse proxy can never forward to it. See issue #33.
     # Binds 0.0.0.0 (no 127.0.0.1 prefix) so Pangolin can reach the container.
     ports:
       - "3000:3000"
@@ -47,7 +51,9 @@ services:
     env_file: .env
     environment:
       DB_HOST: db
-      SITE_INTERNAL_URL: http://app:3000/api/v1/internal/bot-config
+      # Internal config fetch goes to the app's UNPUBLISHED internal port (3001),
+      # not the public 3000. Keep the port in sync with the app's INTERNAL_PORT.
+      SITE_INTERNAL_URL: http://app:3001/internal/bot-config
       SITE_PUBLIC_URL: http://app:3000/api/v1/public
       LOG_DIR: /app/bot/logs
     depends_on: