feat: implement WebSocket endpoint for real-time sale notifications

[brightpixel-dev]

Overview

Producers should receive real-time notifications when their beats are purchased. The backend already streams Horizon contract transactions via SSE — wrap this in a WebSocket endpoint so the frontend and mobile can subscribe to events.

Acceptance Criteria

• GET /ws upgrades to WebSocket connection
• Server broadcasts sale events: { type: 'sale', sampleId, tier, buyer, amountXlm, timestamp }
• Producers can subscribe to events for their address specifically
• Auto-reconnect logic on the client side (exponential backoff)
• Graceful connection cleanup on disconnect

Implementation

Use the ws package. Integrate with the existing streamContractTransactions service in stellar.ts. Parse Soroban event data to extract the sale details from the contract event topics.

// Suggested event shape
interface SaleEvent {
  type: 'sale';
  sampleId: number;
  tier: 0 | 1 | 2;
  buyer: string;
  producer: string;
  amountXlm: string;
  timestamp: string;
}

[nonso7]

Can i work on this issue
I am a blockchain developer with experience building decentralized applications, secure smart contracts, and user-focused Web3 products. My background spans smart contract development, protocol integration, security research, and frontend development using modern Web3 technologies. I have contributed to open-source ecosystems, built production-ready dApps, and worked on solutions involving payments, digital assets, and on-chain infrastructure. I am excited about Stellar's mission of enabling fast, low-cost, and accessible financial services, and I look forward to contributing solutions that expand financial inclusion and utility across emerging markets.

[grantfox-oss]

🦊 GrantFox — @nonso7 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #2)
2. Your PR will be reviewed by the Crate-Protocol maintainers

Good luck! Track your progress on GrantFox.