security(relay): minor hardening follow-ups from PR review

- Uniform auth responses remove the session-enumeration oracle: unknown
  session and wrong password now return an identical 401 at the API layer and
  an identical WS handshake/close; desktop-status and the 503 "not connected"
  signal are only exposed after successful auth.
- Session-count cap re-checked after bcrypt.hash so concurrent creates can't
  overshoot maxSessions.
- Relay web client reconnect backoff resets only after a successful auth
  response (an accept-then-close server no longer defeats the backoff).
- idGenerator comment corrected to match the rejection-sampling; drop unused
  recordFailure return value.
- DEPLOY.md: document ALLOWED_ORIGINS for reverse-proxy deployments.

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

macropad-relay/src/utils/idGenerator.ts

@@ -18,7 +18,11 @@ export function generateSessionId(length: number = 12): string {
   let result = '';
 
   while (result.length < length) {
-    // Over-fetch a little to reduce the number of syscalls when rejecting.
+    // Fetch exactly `length` random bytes per pass. Bytes at or above the
+    // rejection threshold are skipped to avoid modulo bias, so a pass may
+    // yield fewer than `length` characters; the outer while then runs another
+    // pass until we have enough. (No over-fetching — each pass draws exactly
+    // `length` bytes.)
     const bytes = randomBytes(length);
     for (let i = 0; i < bytes.length && result.length < length; i++) {
       const byte = bytes[i];