refactor(netflow-db): merge shards one attachment at a time and require full marker coverage

Drop the single merge transaction and its 11-shard limit: the private
temporary file and rename already make the merge atomic. Refuse
completion markers without a five-minute coverage row for every local
bucket, which catches shards built without an explicit end date.
612b092361flamboh committed on 9/27/2026, 4:40:26 AMparente9db5e3
3 files changedLine totals unavailable