feat: add smart contract-based premium snippet marketplace

[Mayorlay]

feat: Smart Contract-Based Premium Snippet Marketplace

Summary

Implements a decentralized marketplace where developers can publish and purchase premium code
snippets with payments processed via Stellar assets and secured through escrow logic.

Changes

Database (scripts/add-marketplace.sql)

• marketplace_listings — snippet listings with price, seller wallet, and status
• marketplace_purchases — purchase records linking buyers to listings
• marketplace_escrow — escrow lock/release records per purchase
• marketplace_audit_logs — immutable on-chain audit trail for all marketplace actions

Types (marketplace.types.ts)

• Interfaces for listings, purchases, escrow, audit logs, and I/O payloads

Repository (marketplace.repository.ts)

• CRUD operations for all four tables using NeonDB serverless SQL

Service (marketplace.service.ts)

• createListing — publishes a snippet with an on-chain memo transaction
• purchaseListing — verifies buyer wallet signature → locks escrow → releases to seller
  atomically
• verifyOwnership — confirms ownership via completed purchase record and release tx hash
• verifyWalletSignature — Ed25519 signature verification using Stellar Keypair
• Graceful fallback to deterministic mock tx hashes when STELLAR_SECRET_KEY is not set
  (testnet-safe)

Testing

• Works on testnet without a live secret key (mock tx hashes derived from SHA-256)
• Set STELLAR_SECRET_KEY to enable real Horizon transactions

▸ Closes #87

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