Trace ids, failure kinds and one line per event in the JSON log (#91)

From Dash0's structured logging guide, what applies here:

- Each JSON line inside a traced span ends with its trace_id and span_id, so a line in Loki leads
  to its trace in Tempo; the access log is written inside its request's span so it has one too.
  The JSON formatter takes no extra fields, so WithTrace appends them to the object it writes.
- A feed or download failure carries error.type (the HTTP status, or dns, redirect_loop,
  timeout, ...) and http.response.status_code, from failure_kind beside explain_failure, so
  failures group by kind without a regex over msg.
- Each event was logged twice: words under ipx::scan and fields under ipx::io. It is now one
  line under ipx::scan with both; the wire copy is at debug, for the admin page's Daemon I/O tab,
  and out of production's log. The healthcheck's status reply stays under ipx::io.
- The access log's ms is duration_ms. The dashboard and the prod-check skill follow.
- error fields are Display with the anyhow chain everywhere, not a mix of Debug and Display.

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

src/web.rs

@@ -2007,15 +2007,17 @@ async fn access_log(req: Request, next: Next) -> Response {
     let resp = tracing::Instrument::instrument(next.run(req), span.clone()).await;
     span.record("http.response.status_code", resp.status().as_u16());
     if !quiet {
-        let ms = started.elapsed().as_millis() as u64;
+        let duration_ms = started.elapsed().as_millis() as u64;
         let status = resp.status().as_u16();
         // The same as fields, for the JSON log (#91). Unrouted, a request has no route.
         let route = resp.extensions().get::<MatchedPath>().map(|r| r.as_str().to_owned());
         let method = method.as_str();
+        // Inside the request's span, so the line carries its trace id and leads to its trace.
+        let _in = span.enter();
         if resp.status().is_success() || resp.status().is_redirection() {
-            tracing::info!(target: "ipx::http", method, path, route, status, ms, "{method} {path} -> {status} in {ms}ms");
+            tracing::info!(target: "ipx::http", method, path, route, status, duration_ms, "{method} {path} -> {status} in {duration_ms}ms");
         } else {
-            tracing::warn!(target: "ipx::http", method, path, route, status, ms, "{method} {path} -> {status} in {ms}ms");
+            tracing::warn!(target: "ipx::http", method, path, route, status, duration_ms, "{method} {path} -> {status} in {duration_ms}ms");
         }
     }
     resp