feat: add upstreamModel config for model name mapping

When the upstream LLM server expects a different model name than what
clients send, the gateway now rewrites the 'model' field in the request
body before forwarding.

Config example:
  models:
  - name: "ornith:35b"           # client-facing name
    address: "ornith-predictor:80"
    upstreamModel: "qwen2.5:72b"  # what upstream expects

Fixes 400 'model is required' errors when upstream model names differ.

internal/proxy/router.go

@@ -122,6 +122,20 @@ func (h *Handler) routeByModel(r *http.Request, path string) (*Route, error) {
 		}
 	}
 
+	// If upstream expects a different model name, rewrite the body
+	if modelUpstream.UpstreamModel != "" && modelUpstream.UpstreamModel != modelName {
+		payload["model"] = modelUpstream.UpstreamModel
+		newBody, err := json.Marshal(payload)
+		if err != nil {
+			return nil, &modelValidationError{
+				Kind:    "invalid_request",
+				Message: fmt.Sprintf("failed to rewrite model name: %v", err),
+			}
+		}
+		r.Body = io.NopCloser(bytes.NewReader(newBody))
+		r.ContentLength = int64(len(newBody))
+	}
+
 	// Determine the upstream path based on the request path
 	upstreamPath := path
 	if path == "/v1/rerank" {