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))

internal/provider/provider.go

@@ -988,6 +988,7 @@ func (p *Provider) Image(ctx context.Context, req *types.ImageGenRequest) (*type
 	unified, terr := p.vm.Transform(p.adapter+"_image", "transform_response", raw)
 	if terr == nil && unified != raw {
 		if err := json.Unmarshal([]byte(unified), &out); err == nil {
+			out.Model = model // actual model served (AUTO resolved to bestImageModel)
 			p.RecordSuccess(model)
 			return &out, nil
 		}
@@ -1000,6 +1001,7 @@ func (p *Provider) Image(ctx context.Context, req *types.ImageGenRequest) (*type
 		return nil, fmt.Errorf("unmarshal image response: %w", err)
 	}
 	out.ImageData = img.Data
+	out.Model = model // actual model served (AUTO resolved to bestImageModel)
 	p.RecordSuccess(model)
 	return &out, nil
 }

internal/types/types.go

@@ -71,6 +71,7 @@ type ToolCall struct {
 // ---- Unified internal representation (what adapters produce) ----
 
 type UnifiedResponse struct {
+	Model            string     `json:"model,omitempty"`
 	Content          string     `json:"content"`
 	ReasoningContent string     `json:"reasoning_content,omitempty"`
 	FinishReason     string     `json:"finish_reason,omitempty"`