fix(gateway): record actual served model for image requests

handleImage recorded the raw request model id, so AUTO image
generations showed up as model=AUTO in the request records and
by-model aggregates instead of the image model actually served
(e.g. Kwai-Kolors/Kolors).

UnifiedResponse gains an optional Model field; Provider.Image fills
it with the resolved id (AUTO resolves to the source's best image
model), and handleImage prefers it when writing the audit record.

internal/gateway/chat.go

@@ -932,6 +932,9 @@ func (g *Gateway) handleImage(w http.ResponseWriter, r *http.Request) {
 	if usedSrc != "" {
 		rec.Source = usedSrc
 	}
+	if resp.Model != "" {
+		rec.Model = resp.Model // record the actual model served, not the raw request id
+	}
 	rec.OK = true
 	rec.Status = http.StatusOK
 	rec.Compl = int64(len(resp.ImageData))