Build the NestJS Stellar Service to Anchor and Verify Document Hashes on the Stellar Network

[mftee]

Description:
Wrap the stellar-sdk dependency into a standalone NestJS service that can anchor a document hash to the Stellar testnet/mainnet and return the transaction reference.

Tasks:

• Create src/stellar/stellar.module.ts and stellar.service.ts
• Implement anchorHash(hash: string): Promise<{ txHash: string, ledger: number }> using stellar-sdk: build and submit a manageData operation with the document hash as memo or data entry
• Read STELLAR_SECRET_KEY, STELLAR_HORIZON_URL, STELLAR_NETWORK from ConfigService
• Implement verifyHash(hash: string): Promise<boolean> — query Horizon to check if a hash has been anchored
• Export StellarService for use in other modules

Acceptance Criteria:

• anchorHash submits a transaction to Stellar testnet and returns a valid tx hash
• verifyHash returns true if the hash exists on-chain, false otherwise
• Network errors are caught and thrown as typed NestJS exceptions