feat: Implement SEP-40 swap rates endpoint

[dev-susa]

Description

Implement SEP-40 to provide a standardized way for wallets to request real-time swap rates for on-chain asset pairs managed by the anchor.

Changes

• Controller: Added with methods to:

  • Get swap rates for specified asset pairs
  • Retrieve all supported asset pairs
  • Update swap rates (for testing/admin)

• Routes: Added two endpoints:

  • POST /sep40/rates - Get swap rates for specified pairs
  • GET /sep40/pairs - Get all supported asset pairs

• Tests: Comprehensive unit and integration tests covering:

  • Valid and invalid asset pairs
  • Case-insensitive asset codes
  • Inverse rate calculations
  • Error handling

• Documentation: Added detailed SEP-40 documentation with:

  • API endpoint specifications
  • Usage examples
  • Implementation details
  • Future enhancement suggestions

Supported Assets

• XLM (Stellar Lumens)
• USDC (USD Coin)
• USDT (Tether)
• BTC (Bitcoin)
• ETH (Ethereum)

Testing

All tests pass:

npm test -- sep40.controller.test.ts
npm test -- sep40.route.test.ts

Closes

Closes #229

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[Ceejaytech25]

Nice implementation, lgtm!