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  bot's unpublished port-4100 pattern.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  document that INTERNAL_PORT stays unpublished.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

bot/.env.example

@@ -24,9 +24,10 @@ LOG_TO_FILE=true        # set false for console-only
 BOT_INTERNAL_KEY=dev-only-change-me-bot-key
 
 # Where this bot calls back to the main site to fetch its config on boot
-# (GET .../api/v1/internal/bot-config), so a restart self-reconnects without
-# needing the admin panel to push config again.
-SITE_INTERNAL_URL=http://localhost:3000/api/v1/internal/bot-config
+# (GET .../internal/bot-config), so a restart self-reconnects without needing
+# the admin panel to push config again. This targets the site's UNPUBLISHED
+# internal port (INTERNAL_PORT, default 3001) — NOT the public 3000. See #33.
+SITE_INTERNAL_URL=http://localhost:3001/internal/bot-config
 
 # Read-only PUBLIC API base (Phase 7) — no shared secret, same data any
 # visitor's browser can fetch. Used by /wiki (search) and /announce