feat(backend): integrate Stellar transaction history via Horizon API

[nonchalanttee22-lgtm]

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Summary

This PR adds transaction history fetching and display for Stellar transactions associated with contracts, giving clients and freelancers transparency and traceability into on-chain activity. Transactions are fetched from the Horizon API, mapped to contracts, exposed via a paginated endpoint, and returned in a UI-ready schema.

Closes #128

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What changed

Backend Service

• Implemented a service that fetches transactions from the Horizon API for a given Stellar account/contract
• Each transaction is mapped to its corresponding contract record in the database, enabling lookup by contract ID
• Added explicit Testnet support — configurable network selection (Testnet/Mainnet) for development and testing without code changes
• Added error handling for failed Horizon queries (timeouts, rate limits, malformed responses) with clear, actionable error messages surfaced to the API layer rather than raw Horizon errors

API Endpoint

• New endpoint returning transaction history for a contract, with pagination support (cursor or page-based, matching existing API conventions) to handle large transaction sets performantly
• Response schema is UI-ready and includes per transaction:
  • hash — unique transaction identifier
  • timestamp — block time of the transaction
  • amount — asset type and value
  • status — reflects actual on-chain state (successful, failed, pending)

Status Mapping

• Status field is derived directly from the Stellar network's transaction result, not inferred or cached, ensuring it accurately reflects real-time on-chain state

Tests

• Unit tests for the Horizon-fetching service, including mocked success and failure responses
• Tests for contract-to-transaction mapping logic
• Tests for the paginated API endpoint, including boundary cases (empty result set, single page, multi-page)
• Tests confirming Testnet transactions are fetched and displayed correctly

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How to verify

npm test -- --grep transactionHistory

• Fetch transaction history for a contract with known Testnet transactions and assert hash, timestamp, amount, and status are all correctly populated
• Request a large transaction set and assert pagination returns consistent, performant results across pages
• Simulate a Horizon API failure (timeout/error response) and assert a clear, descriptive error is returned rather than a raw exception
• Assert transaction status in the response matches the actual state reported by the Stellar network (successful/failed/pending)
• Switch network config to Testnet and confirm transactions resolve correctly in a dev/test environment

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Checklist

• Transaction history fetches correctly for contracts via Horizon API
• Transactions mapped to associated contracts in the database
• API endpoint returns paginated, performant results
• Testnet supported and verified for development/testing
• Status field reflects real-time transaction state from the Stellar network
• Clear error messages returned on failed Horizon queries
• Response schema includes hash, timestamp, amount, and status — UI-ready
• Unit tests pass for backend logic and API responses
• Closes [Feature]: Stellar Transaction History Integration #128

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