feat(api): add request/response schema validation with problem+json and OpenAPI generation (#998)

[yodakkak]

Closes #998

Summary

• Added �pi/middleware/schemaValidation.js enforcing declarative request (params, query, �ody) and response schemas, returning RFC 9457 �pplication/problem+json responses on validation failure and generating OpenAPI 3.1 specifications directly from route schemas.
• Wired route schemas into �pi/routes/accounts.js and �pi/routes/transactions.js.
• Added unit tests in ests/api/schemaValidation.test.js covering valid, boundary, and malformed payloads plus OpenAPI spec generation.

---

Payout Wallet (Stellar / USDC): GD3TVG3SLNQMK7VS7772FZPNM5LTH24NR5NSWMURN2CXEJUDA4V2XONU
EVM Wallet: 0x59622Fe8f9b438eF1CD5dB4e91f1230fe6239AC9

[vercel]

@yodakkak is attempting to deploy a commit to the nanle-code's projects Team on Vercel.

A member of the Team first needs to authorize it.