docs: complete API error codes, deploy troubleshooting, monitoring/security rewrite, L2+L3 building blocks

DEPLOYMENT.md

@@ -228,5 +228,19 @@ docker compose -f docker-compose.prod.yml down -v
 - Missing frontend build artifacts in local-only compose workflows.
 - Wrong registry credentials or missing login.
 - Misaligned domain env vars (app/ws/hls) causing mixed-origin errors.
+
+## Troubleshooting
+
+| Symptom                    | Check                                         | Fix                                                           |
+| -------------------------- | --------------------------------------------- | ------------------------------------------------------------- |
+| Service not starting       | `docker logs linearvoid-<svc> --tail 50`      | Check `.env` vars, volume mounts, port conflicts              |
+| Playout stall / slate mode | `curl http://<host>:8081/health` — mode=slate | Check playlist file, media paths, disk space                  |
+| Chat not connecting        | `curl http://<host>:3000/health`              | Check Redis: `docker exec linearvoid-redis redis-cli ping`    |
+| Admin 401 loop             | Cookie not sent                               | Verify `X-Forwarded-Proto: https`, nginx auth_request headers |
+| Upload fails (413)         | File too large                                | Check nginx `client_max_body_size` (should be 64m+)           |
+| HLS not playing            | `curl http://<host>/hls/playlist.m3u8`        | Check playout health, HLS dir permissions                     |
+| Prometheus no targets      | `curl http://<host>:9090/targets`             | Verify container network, scrape config                       |
+| Grafana no data            | Check datasource                              | Verify Prometheus URL `http://prometheus:9090` provisioned    |
+
 - Proxy-layer body size limits returning 413 on upload.
 - Health checks failing before service warmup is complete.