Record cancellation from what the reverse proxy observed, not the request context

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/proxy/forward.go

@@ -54,7 +54,7 @@ func (p *Handler) forward(w http.ResponseWriter, r *http.Request, route, host, l
 	total := time.Since(started)
 
 	req := forwardRow(route, fp, model, host, started, waited, rev, rec.status, total.Milliseconds())
-	if r.Context().Err() != nil {
+	if rev.cancelled {
 		req.Status = 499
 		req.Err = "client cancelled"
 	}
@@ -126,6 +126,9 @@ type forwardState struct {
 	ttfb     time.Time
 	streamed bool
 	tee      *tee
+	// cancelled is set when the reverse proxy's ErrorHandler observed the client leaving before a
+	// response byte was written; it is the one signal that turns a delivered row into a 499.
+	cancelled bool
 }
 
 // statusRecorder records the status written and forwards Flush so the reverse proxy can stream.
@@ -215,6 +218,9 @@ func newReverseProxy(h Health, host, leaseState, ctxHeader string, target *url.U
 		},
 		ErrorHandler: func(w http.ResponseWriter, req *http.Request, err error) {
 			if errors.Is(err, context.Canceled) {
+				// The client left before any byte was written; record it so the row is a 499,
+				// not the post-hoc context check that misread a pooled close as a cancel.
+				rev.cancelled = true
 				return
 			}
 			h.MarkDown(host, err.Error())