feat: add GET /utils/keypair endpoint (#81)

[utilityjnr]

Closes #81

What was done

Added a new utility endpoint GET /utils/keypair that generates a fresh random Stellar keypair using the official Stellar SDK.

How it was implemented

Endpoint: GET /utils/keypair

File: src/routes/utils.js

• Imported Keypair from @stellar/stellar-sdk (added to the existing destructured import alongside Transaction and Networks).
• Added a new route handler for GET /keypair on the utils router.
• The handler reads STELLAR_NETWORK from the environment (defaulting to 'testnet') and throws a 403 error if the network is not 'testnet', matching the same testnet-guard pattern used by the existing GET /utils/friendbot/:accountId endpoint.
• On testnet, it calls Keypair.random() to generate a cryptographically random Ed25519 keypair and returns the public key, secret key, and a safety warning via the shared success() response helper.

Response shape:
{
"success": true, "data": { "publicKey": "G...", "secretKey": "S...", "warning": "Never share your secret key" } }

Tests: tests/utils.keypair.test.js

Three test cases added using Jest + Supertest:

1. Returns a valid keypair on testnet — verifies HTTP 200, success flag, public key matches /^G[A-Z2-7]{55}$/, secret key matches /^S[A-Z2-7]{55}$/, and warning message is present.
2. Returns a different keypair on each call — calls the endpoint twice and asserts both keys differ, confirming randomness.
3. Returns 403 on mainnet — sets STELLAR_NETWORK=mainnet and asserts the endpoint returns 403 with success: false.

All 3 tests pass.

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