Integrations docs

External-service wiring and debug notes.

• websocket-debugging.md – WebSocket 400 error debugging
• fcm-troubleshooting.md – FCM token retrieval troubleshooting

For the auth subsystem (JWT, rotating refresh tokens, password reset, token storage) see ../architecture/auth/. For the end-to-end notification system (delivery, throttling, inbox, timezone rules) see ../features/notifications/.