Add structured logging and better error handling

README.md

@@ -64,7 +64,14 @@ Every first level folder inside these roots is a move destination:
 | `DOWNLOAD_DIR` | `/data/downloads` | Temporary download directory |
 | `MIN_FREE_GB` | `10` | Initial value: free space that must always remain |
 | `MAX_CONCURRENT_DOWNLOADS` | `2` | Initial value: parallel downloads, others wait in queue |
+| `CONNECTIONS_PER_DOWNLOAD` | `4` | Initial value: parallel connections for one download when the server allows ranges |
 | `ALLOWED_PORTS` | `80,443` | Initial value: allowed remote ports, `*` for any |
+| `LOG_LEVEL` | `info` | `debug`, `info`, `warn` or `error`. `debug` also logs every polling request |
+| `LOG_FORMAT` | `text` | `text` or `json` |
+
+## Logging
+
+Logs go to stdout (`docker compose logs -f`). Every request gets an `X-Request-ID`. When the server hits an internal error, the page shows a reference ID and the log line for that request carries the same `request_id`, so the two can be matched. Download, extraction and move steps are logged with a short `download` ID.
 
 ## Web Settings