fix(api): statusWriter implements http.Flusher for SSE handlers

The logMiddleware wrapper (statusWriter) didn't forward Flush, so
`w.(http.Flusher)` in SSE handlers failed and returned the plain text
"streaming unsupported" with 500. SSE clients (agents_dashboard C++ app)
saw a closed connection with no events.

Add Flush() that delegates to the embedded ResponseWriter when it
implements Flusher. Required for /sse/status and /sse/agents/{id}/logs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/server.go

@@ -147,6 +147,16 @@ func (sw *statusWriter) WriteHeader(code int) {
 	sw.ResponseWriter.WriteHeader(code)
 }
 
+// Flush forwards to the underlying ResponseWriter when it implements Flusher.
+// Without this method, the type assertion `w.(http.Flusher)` in the SSE handlers
+// fails (the wrapper hides the inner Flusher), and the handler aborts with
+// "streaming unsupported".
+func (sw *statusWriter) Flush() {
+	if f, ok := sw.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
 // --- Helpers ---
 
 func writeJSON(w http.ResponseWriter, status int, v any) {