collect: revert last_broadcast to snapshot, not payload

Diffs should always be computed against the last full snapshot,
not the last broadcast payload.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/collect.py

@@ -93,7 +93,7 @@ def run(args: argparse.Namespace) -> None:
                         payload = jay_diff_full(last_broadcast, snapshot, combine_upd_add=True)
                     else:
                         payload = snapshot
-                    last_broadcast = payload
+                    last_broadcast = snapshot
                     network.broadcast(payload, push_clients, push_lock)
                     log.info("Record #%d saved at %s", len(store["records"]), snapshot["ts"])
                 except KeyboardInterrupt: