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 }