fix: langfuse v4 SDK compatibility and pass user message as trace input

services/chat/app/main.py

@@ -31,6 +31,11 @@ logging.getLogger("sqlalchemy.pool").setLevel(logging.WARNING)
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    # Initialise Langfuse tracing (no-op if keys are missing)
+    from app.tracing import init_langfuse
+
+    init_langfuse()
+
     # Start Redis consumer in background
     from app.redis_consumer import start_consumer