Review fixes: record cancelled requests as 499; empty usage is an array

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

internal/proxy/proxy.go

@@ -249,6 +249,16 @@ func (p *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		p.log.Warn("request", "route", route, "host", host, "method", r.Method, "path", rest, "status", 499)
+		p.writeRecord(store.Request{
+			Route:   route,
+			FP:      fp,
+			Model:   model,
+			Host:    host,
+			Started: started,
+			TotalMs: time.Since(started).Milliseconds(),
+			Status:  499,
+			Err:     "client cancelled while queued",
+		})
 		return
 	}
 	defer release()