Log as JSON when IPX_LOG_FORMAT=json (#91)

The log was text, so the Grafana dashboard picked lines apart with
regular expressions, and a change of wording would have blanked its
panels. With IPX_LOG_FORMAT=json each line is one JSON object: the
access log carries method, path, route, status and ms as fields (the
route passed from the routing layer in the response's extensions), and
each wire event its ev, feed, new, downloaded, failed, bytes, msg and
the rest (log_wire), beside the old message. The two startup lines that
were println! are logged, so no line breaks the JSON. Text stays the
default, for a terminal. The dashboard reads the fields with Loki's json
parser, and groups requests by route rather than path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/configuration.md

@@ -142,6 +142,7 @@ and they are re-derived on every scan. Editing one in the UI promotes it to a ca
 | `IPX_DATABASE_URL` | A `postgres://user:password@host:port/database` URL: use that database instead of `state.db` |
 | `IPX_TEST_DATABASE_URL` | For `cargo test`: run the database tests on this Postgres database too, each in a schema of its own |
 | `IPX_LOG` | What reaches stderr (`ipx=debug`, `ipx::scan=debug`, …) |
+| `IPX_LOG_FORMAT` | `json` for one JSON object a line, with each request's and event's fields as its own (for Loki and the like); text otherwise |
 | `IPX_UI_LOG` | What the in-process log buffer captures for the UI's Log view |
 | `OTEL_EXPORTER_OTLP_ENDPOINT` | An OTLP/HTTP collector, such as Tempo at `http://host:4318`: the daemon sends it traces of scans, downloads and web requests. The other `OTEL_EXPORTER_OTLP_*` variables apply too |
 | `http_proxy` / `https_proxy` | Honoured for feed and enclosure fetches |