fix(gateway): forward Flush through statusRecorder so SSE streams in real time

statusRecorder (the access-log wrapper) did not implement http.Flusher, so
w.(http.Flusher) inside streamChat/streamChatAuto returned nil and every SSE
chunk stayed buffered until the response ended. Add Flush() that delegates to
the underlying writer when it supports flushing.

Regression test: TestStatusRecorderFlusher pins the interface assertion and
the forwarding path.

internal/gateway/gateway_test.go

@@ -671,3 +671,34 @@ func TestHasScopeModelWithSourcePrefix(t *testing.T) {
 		t.Error("AUTO scope should allow any prefixed model")
 	}
 }
+
+// TestStatusRecorderFlusher pins the SSE-critical contract: the access-log
+// wrapper must implement http.Flusher, otherwise the streaming chat handlers'
+// w.(http.Flusher) assertion yields nil and chunks are never flushed until
+// the response ends (regression for the buffered-SSE bug).
+func TestStatusRecorderFlusher(t *testing.T) {
+	var inner *flushRecorder
+	rr := httptest.NewRecorder()
+	inner = &flushRecorder{ResponseWriter: rr}
+	sr := &statusRecorder{ResponseWriter: inner}
+
+	fl, ok := any(sr).(http.Flusher)
+	if !ok {
+		t.Fatal("statusRecorder does not implement http.Flusher — SSE streaming is broken")
+	}
+	if inner.flushed {
+		t.Fatal("Flush called before Flush()")
+	}
+	fl.Flush()
+	if !inner.flushed {
+		t.Fatal("statusRecorder.Flush did not forward to the underlying writer")
+	}
+}
+
+// flushRecorder records whether Flush was forwarded.
+type flushRecorder struct {
+	http.ResponseWriter
+	flushed bool
+}
+
+func (f *flushRecorder) Flush() { f.flushed = true }

internal/gateway/server.go

@@ -113,6 +113,17 @@ func (s *statusRecorder) Write(b []byte) (int, error) {
 	return s.ResponseWriter.Write(b)
 }
 
+// Flush forwards to the underlying writer so SSE handlers can stream
+// incrementally through the access-log wrapper. Without this method the
+// w.(http.Flusher) assertion inside the streaming chat handlers fails (the
+// embedded ResponseWriter interface does not carry Flush into the method set),
+// and every chunk stays buffered until the response ends.
+func (s *statusRecorder) Flush() {
+	if f, ok := s.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
 func (g *Gateway) routes(w http.ResponseWriter, r *http.Request) {
 	switch {
 	case r.URL.Path == "/v1/chat/completions":