Fintech engineering

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36import { createPublicClient, webSocket, parseAbiItem } from 'viem';const client = createPublicClient({ transport: webSocket(RPC_WS) });/** * Watches for settlement events. The hard part of chain integration is not * subscribing — it is that a block you already acted on can be reorganised out * from under you, and that your node will drop the socket at 03:00. */client.watchEvent({  address: SETTLEMENT_CONTRACT,  event: parseAbiItem('event Settled(bytes32 indexed ref, address payer, uint256 amount)'),  onLogs: async (logs) => {    for (const log of logs) {      // Never act on one confirmation. Record it, and let the confirmation      // counter below decide when it is safe to treat as final.      await db.settlement.upsert({        where: { txHash: log.transactionHash },        create: { ref: log.args.ref, amountMinor: log.args.amount, confirmations: 1 },        update: { confirmations: { increment: 1 }, removed: false },      });    }  },  // A reorg replays logs with removed: true. Anything already released has to be  // walked back, which is why release is a separate step from observation.  onError: (error) => alerts.page('chain-listener', error),});// The socket will drop. Reconnect, then backfill the gap from the last block we// definitely processed — resubscribing alone loses everything in between.client.onDisconnect(async () => {  const from = await db.cursor.get('settlement');  await backfill(from, await client.getBlockNumber());});