[BE-60] Implement Stellar SDK service — connect NestJS backend to deployed Soroban contracts

[yusuftomilola]

Overview

The Stellar smart contracts (contracts/assetsup/) are written and tested but the backend has no code to interact with them. This issue creates the StellarModule — the NestJS wrapper around the Stellar SDK that all blockchain feature modules will use.

Context

• package.json has @stellar/stellar-sdk@14.5.0 installed but never imported
• The deployed contract address will come from STELLAR_CONTRACT_ID env var
• The backend needs a funded Stellar keypair to sign and submit transactions: STELLAR_SECRET_KEY env var
• The contracts source is at contracts/assetsup/src/lib.rs — review exported functions before implementing

Acceptance Criteria

• Add env vars to backend/.env.example: STELLAR_NETWORK (testnet|mainnet), STELLAR_RPC_URL, STELLAR_SECRET_KEY, STELLAR_CONTRACT_ID, STELLAR_NETWORK_PASSPHRASE
• Create StellarModule and StellarService using @stellar/stellar-sdk
• StellarService.buildKeypair() — loads keypair from STELLAR_SECRET_KEY
• StellarService.simulateTransaction(contractId, method, args) — uses Soroban RPC simulateTransaction, returns decoded result without submitting
• StellarService.submitTransaction(contractId, method, args) — builds the Soroban transaction, signs with keypair, submits, polls for confirmation, returns { txHash, ledger, result }
• StellarService.getContractData(contractId, key) — reads on-chain storage for a given data key
• All Stellar errors are caught and wrapped in NestJS ServiceUnavailableException with a descriptive message

[adeboladee]

@adeboladee has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, Please I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @adeboladee to this issue.

[goodness-cpu]

@goodness-cpu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer, I can fix this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @goodness-cpu to this issue.

[NickiM84]

@NickiM84 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, I'm a fullStack developer with years of experience, I have worked on several projects, can I please work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @NickiM84 to this issue.

[RUKAYAT-CODER]

@RUKAYAT-CODER has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, am a full stack developer. can i take up this task?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @RUKAYAT-CODER to this issue.

[drips-wave]

Congratulations, @RUKAYAT-CODER! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @RUKAYAT-CODER: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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