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/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"`