security: Sprint 00 — critical security fixes (OPC WS auth, Transmission creds, SECRET_KEY, passkey rate limit)

- Add JWT token auth to OPC WebSocket (unauthenticated → 403, per-user tracking)
- Externalize Transmission RPC credentials to TRANSMISSION_USER/PASS env vars
- Remove hardcoded SECRET_KEY fallback from dev compose
- Rate-limit passkey register options endpoint at 5/minute
- Add PDD (docs/improvement-plan.md) and sprint specs (specs/)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dashboard/docker-compose.yml

@@ -53,6 +53,8 @@ services:
       - LITELLM_HEALTH_API_KEY=${LITELLM_HEALTH_API_KEY:-}
       - LITELLM_URL=http://litellm:4005
       - GITEA_DB_PASSWORD=${GITEA_DB_PASSWORD}
+      - TRANSMISSION_USER=${TRANSMISSION_USER}
+      - TRANSMISSION_PASS=${TRANSMISSION_PASS}
     volumes:
       - /volume1:/volume1
       - /volume1/docker/nas-dashboard/ssh/dashboard_terminal:/app/ssh/id_ed25519:ro