feat: cursor pagination, standardized error DTO, and Zod hardening

[Emeka000]

Summary

• Add cursor pagination to GET /orders #562 Cursor pagination for GET /orders/user/:address — fixed stale offset-pagination tests (skip, total, page) that were mismatched with the existing cursor-based implementation; updated assertions to use nextCursor, hasNext, take: limit+1; added tests for nextCursor generation, invalid cursor (400), and cursor-driven next-page fetch
• Standardize error DTO in global error handler #564 Standardize error DTO — added code field to ErrorResponse (VALIDATION_ERROR, NOT_FOUND, UNAUTHORIZED, FORBIDDEN, INTERNAL_ERROR, BAD_REQUEST); resolveCode() helper in errorHandler.ts; covered by new tests
• Harden order body validation with zod schemas #563 Harden order body validation with Zod — replaced z.string().min(1) on userAddress with STELLAR_PUBLIC_KEY_REGEX so malformed Stellar keys are caught at the Zod layer before any DB call; added zod to dependencies

Test plan

• GET /orders/user/:address — response shape: nextCursor, hasNext, limit; no total or page
• GET /orders/user/:address?limit=2 with 3 DB rows → hasNext: true, nextCursor is a string, orders.length === 2
• GET /orders/user/:address?cursor=<invalid> → 400
• GET /orders/user/:address?cursor=<valid base64url> → 200, applies cursor WHERE clause
• GET /orders/user/:address?status=OPEN → findMany called with take: 21, no skip
• POST /orders with non-Stellar userAddress → 400 from Zod before any DB call
• Any error response contains { error, code, requestId, statusCode } with correct code per error class

Note: All 32 test suites currently fail in CI due to a pre-existing Prisma schema error (ResolutionCandidate missing back-relation on Market) that prevents prisma generate. This blocks the entire test runner and is unrelated to these changes. TypeScript compilation passes cleanly on all modified files.
closes #561

closes #563

closes #564

closes #562