fix(api): add SSE headers to prevent buffering & enable real-time streaming

SSE events in the Next.js 15 client were arriving in bursts (UI updated all at once) while Vite streamed fine.

Root cause: intermediaries (Next proxy/CDN/Nginx) were buffering or transforming the response.

Add explicit streaming-friendly headers on the SSE endpoint:

- Cache-Control: no-cache, no-transform → disable caching and proxy transformations (e.g., gzip) that break chunked delivery.
- Connection: keep-alive → keep the HTTP connection open for long-lived SSE.
- X-Accel-Buffering: no → disable Nginx proxy buffering so chunks flush immediately.

Author: asaf

packages/api/src/flux0_api/sessions.py

@@ -631,6 +631,11 @@ async def create_event_and_stream(
                     recording_store,
                 ),
                 media_type="text/event-stream",
+                headers={
+                    "Cache-Control": "no-cache, no-transform",
+                    "Connection": "keep-alive",
+                    "X-Accel-Buffering": "no",
+                },
             )
         # elif params.source == EventSourceDTO.AI_AGENT:
         #     return await _add_agent_message(session_id, params)