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/config/config.go

@@ -36,6 +36,10 @@ type ModelUpstream struct {
 	Address string
 	// Path is the upstream path for this model (e.g., "/v1/chat/completions").
 	Path string
+	// UpstreamModel is the model name to send to the upstream server.
+	// If empty, the client-provided model name (Name) is used as-is.
+	// Use this when the upstream expects a different model name than clients send.
+	UpstreamModel string
 	// AuthRequired indicates whether this model requires JWT authentication.
 	AuthRequired bool
 }