fix(middleware): pass through http.Flusher for SSE streaming (#52)

The request-logging, metrics, and tracing middlewares each wrap the
ResponseWriter to capture status/bytes, but none implemented
http.Flusher. Streaming handlers (/v1/answer/treewalk?stream=true,
/v1/query/stream) do w.(http.Flusher) and errored with 'response
writer does not support http.Flusher'. Add Flush() (delegating to the
wrapped writer) + Unwrap() to statusRecorder, metricsRecorder, and
tracingRecorder so flushes propagate through the whole chain and SSE
streams in real time.

Author: hallelx2

internal/middleware/logging.go

@@ -28,6 +28,18 @@ func (r *statusRecorder) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// Flush propagates to the wrapped writer so SSE/streaming handlers can
+// flush through this middleware. Without it, w.(http.Flusher) fails and
+// streaming endpoints error out.
+func (r *statusRecorder) Flush() {
+	if f, ok := r.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
+// Unwrap exposes the wrapped writer for http.ResponseController.
+func (r *statusRecorder) Unwrap() http.ResponseWriter { return r.ResponseWriter }
+
 // AccessLog logs one structured line per HTTP request: method, path,
 // status, duration, request ID, and principal ID.
 //

internal/middleware/metrics.go

@@ -56,6 +56,17 @@ func (r *metricsRecorder) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// Flush propagates to the wrapped writer so SSE/streaming handlers can
+// flush through the metrics middleware.
+func (r *metricsRecorder) Flush() {
+	if f, ok := r.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
+// Unwrap exposes the wrapped writer for http.ResponseController.
+func (r *metricsRecorder) Unwrap() http.ResponseWriter { return r.ResponseWriter }
+
 // normalizePath strips URL parameters to keep metric cardinality
 // bounded. Document and section IDs are replaced with a placeholder.
 func normalizePath(path string) string {

internal/middleware/tracing.go

@@ -67,6 +67,17 @@ func (r *tracingRecorder) WriteHeader(code int) {
 	r.ResponseWriter.WriteHeader(code)
 }
 
+// Flush propagates to the wrapped writer so SSE/streaming handlers can
+// flush through the tracing middleware.
+func (r *tracingRecorder) Flush() {
+	if f, ok := r.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
+// Unwrap exposes the wrapped writer for http.ResponseController.
+func (r *tracingRecorder) Unwrap() http.ResponseWriter { return r.ResponseWriter }
+
 // TraceIDFromContext extracts the trace ID string from the current span,
 // useful for correlating logs with traces.
 func TraceIDFromContext(ctx context.Context) string {