security(relay): harden internet-facing relay server

Closes the account-takeover chain and related issues found in audit:

- WS auth brute-force protection: per-session lockout (authThrottle) + per-
  socket failure cap that closes the socket (1008); applied to both web-client
  and desktop auth. Failed auth no longer leaves the socket open for unlimited
  guesses.
- WS upgrades now pass an IP-based rate limiter and Origin allowlist before
  handleUpgrade (previously bypassed all HTTP middleware); trust proxy set so
  limiting keys on the real client IP.
- /health no longer leaks live session IDs (counts only).
- .env.example rate-limit fixed (900000ms / 300) from the accidental ~11k rps.
- Credentials moved out of URLs into the X-MacroPad-Password header; images
  fetched via header + blob URLs; login stores creds in sessionStorage.
- Session creation bounded (max sessions, min password length) and TTL-pruned;
  session store writes are now atomic (temp+rename) and debounced.
- Session IDs lengthened to 12 chars with rejection sampling (no modulo bias).
- Enable helmet CSP; restrict CORS to configured origins.
- Request IDs use crypto.randomUUID; drop postinstall build hook and uuid dep.
- Uniform response for unknown session IDs (removes enumeration oracle).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

macropad-relay/src/handlers/apiProxy.ts

@@ -18,9 +18,9 @@ export function createApiProxy(
       return res.status(404).json({ error: 'Session not found' });
     }
 
-    // Check password in header or query
-    const password = req.headers['x-macropad-password'] as string ||
-                     req.query.password as string;
+    // Credentials must be supplied via the header (never the URL/query,
+    // which leaks into logs, history and referrers).
+    const password = req.headers['x-macropad-password'] as string;
 
     if (!password) {
       return res.status(401).json({ error: 'Password required' });