refactor(api): _ndjson_line helper for streaming responses

backend/src/cyclone/api.py

@@ -60,6 +60,17 @@ from cyclone.store import (
 log = logging.getLogger(__name__)
 
 
+def _ndjson_line(event: dict) -> bytes:
+    """Serialize one event dict as a single NDJSON line (UTF-8, trailing ``\\n``).
+
+    Used by the live-tail streaming endpoints to emit a uniform wire format
+    that the frontend ``tail-stream.ts`` parser can split on newlines.
+    Compact separators keep each line small and avoid ambiguity with embedded
+    whitespace.
+    """
+    return (json.dumps(event, separators=(",", ":")) + "\n").encode("utf-8")
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncIterator[None]:
     """Initialize per-process resources (DB + EventBus) before the app