fix(logging): implement http.Flusher on responseWriter for SSE streaming

The logging middleware's responseWriter wrapped http.ResponseWriter but
only implemented WriteHeader, Write, and Unwrap. The missing Flush()
method caused w.(http.Flusher) type assertions to fail in the claudebox
sidecar's streaming endpoint, returning 500 "streaming not supported".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/logging/middleware.go

@@ -33,6 +33,13 @@ func (rw *responseWriter) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// Flush implements http.Flusher, required for SSE streaming through middleware chains.
+func (rw *responseWriter) Flush() {
+	if f, ok := rw.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
 // Unwrap returns the original http.ResponseWriter, required for http.ResponseController.
 func (rw *responseWriter) Unwrap() http.ResponseWriter {
 	return rw.ResponseWriter