🚧 Update topic explorer

backend/app/routers/topics.py

@@ -116,7 +116,10 @@ async def stream(session_id: str, user: OperatorRole):
         raise HTTPException(status.HTTP_404_NOT_FOUND, "session not found") from exc
 
     async def event_stream():
-        index = 0
+        # Start BELOW the first index: messages_since uses strictly-
+        # greater semantics, and a fresh session's first message has
+        # index 0 — a cursor starting at 0 would drop it.
+        index = -1
         heartbeat = 0.0
         try:
             while True: