feat(web): harden login gate — throttle, scoped TLS, token-derived seal key

Remediates the two web-console residuals from the 2026-07-05 posture audit:

- Brute-force throttle (loginThrottle.ts): per-IP exponential backoff
  after 5 free attempts, plus a global floor for spread-out floods, keyed
  on the socket peer IP (not spoofable X-Forwarded-For) with a size-capped
  map. The constant-time compare already stopped the timing leak; this
  bounds guess *volume* against a by-design LAN-exposed console.
- Session seal key now derives from the high-entropy mgmt token instead of
  the low-entropy login password, so a captured cookie is no longer an
  offline password oracle. Falls back to the password only when no token
  is configured (dev/local). Rotating the token now invalidates sessions.
- Replace the process-wide NODE_TLS_REJECT_UNAUTHORIZED=0 with per-request
  Bun TLS scoped to the loopback proxy hop; a non-loopback mgmt URL now
  verifies normally. Dropped the env var from the systemd unit, Steam Deck
  installer, Windows run scripts, env examples, and web README.

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

scripts/windows/web-run.cmd

@@ -37,7 +37,8 @@ rem Fixed deployment wiring (the Windows analogue of scripts/punktfunk-web.servi
 set "PORT=47992"
 set "HOST=0.0.0.0"
 set "PUNKTFUNK_MGMT_URL=https://127.0.0.1:47990"
-set "NODE_TLS_REJECT_UNAUTHORIZED=0"
+rem No NODE_TLS_REJECT_UNAUTHORIZED: the host's self-signed cert is accepted only for the loopback
+rem proxy hop, scoped inside the proxy code (Bun per-request TLS), not process-wide.
 rem Serve HTTPS (HTTP/1.1 over TLS) with the host's identity cert; mark the session cookie Secure.
 set "PUNKTFUNK_UI_TLS_CERT=%CERTFILE%"
 set "PUNKTFUNK_UI_TLS_KEY=%KEYFILE%"