feat: implement contract address registry API

[meshackyaro]

Description

This PR implements a Contract Address Registry API that exposes deployed Soroban contract addresses through a centralized endpoint, enabling hot-swappable contract deployments without frontend rebuilds.

Changes

Backend

• Added GET /api/contracts endpoint that returns contract addresses with metadata
• Implemented configuration service that reads from environments.toml with automatic fallback to environment variables
• Added contract validator for Stellar address format validation
• Integrated structured logging for all contract registry operations

Frontend

• Created contracts.ts service with automatic caching (1-hour TTL)
• Implemented retry logic with exponential backoff (3 attempts: 1s, 2s, 4s)
• Added TypeScript type definitions for type-safe contract access
• Included migration guide for transitioning from hardcoded addresses

Configuration

• Updated environments.toml with sample contract data for testnet and mainnet
• Supports dynamic contract discovery (add new contracts via config only)

Documentation

• Comprehensive API documentation in docs/CONTRACT_REGISTRY_API.md
• Full spec with requirements, design, and implementation tasks

Acceptance Criteria

• ✅ Endpoint returns structured JSON with contractId, network, and version per contract
• ✅ Values sourced from environments.toml or server-side env vars (not hard-coded)
• ✅ Frontend contracts.ts service fetches and caches the registry on startup
• ✅ Adding a new contract requires only a config change, not a code change
• ✅ Response includes deployedAt ledger sequence for reference

Testing

Start the backend and test the endpoint:

cd backend
npm run dev

# Test endpoint
curl http://localhost:3000/api/contracts

Supported Contracts

• bulk_payment
• vesting_escrow
• revenue_split
• cross_asset_payment

Closes #151

[meshackyaro]

Hi @Wilfred007, PR created and pipeline fixed