fix: derive immutable chunk size from network security parameter
chunk_for_slot hardcoded 21600 slots per chunk, which only holds for mainnet and preprod (k = 2160). Preview chunks span 4320 slots (k = 432), so create-snapshots compared incompatible chunk numbers, wrongly concluded the local cardano-db covered the target slots and skipped the Mithril download, then failed to store the ledger snapshot.