Feat : add Contract Address Registry API

[ryzen-xp]

Pull Request Description

Summary

Adds a new GET /api/contracts endpoint to expose deployed Soroban contract addresses for testnet and mainnet.

Changes

• Implemented ContractRegistryService to load contract data from environments.toml
• Added ContractRegistryController to handle API responses
• Added /api/contracts route
• Supports optional ?network= query parameter
• Response includes contractId, network, version, and deployedAt
• Added caching headers for performance

Purpose

This allows the frontend to fetch and cache contract addresses at startup instead of relying on hardcoded environment variables. Contract deployments can now be updated via configuration changes without requiring a frontend rebuild.

Acceptance Criteria Covered

• Structured JSON response per contract
• Data sourced from configuration (no hardcoding)
• Supports multi-network (testnet/mainnet)
• Includes deployed ledger sequence (deployedAt)
• New contracts can be added via config only

Close #79