[WallexCoder]

Summary

Implements the 3D Secure (3DS) card-payment authentication flow requested in issue #925.

What's added

File	Purpose
backend/src/services/3ds.ts	Core service: session lifecycle (initiate, verify, cancel, purge), in-memory store, BIN-based enrolment heuristic, HMAC auth-value generation
backend/src/schemas/3ds.ts	Zod schemas + TypeScript types for all four endpoints
backend/src/routes/3ds.ts	Express router wiring validation → service
backend/src/index.ts	Mounts the new router under /api/v1/3ds
backend/src/services/__tests__/3ds.test.ts	24 service unit tests
backend/src/routes/__tests__/3ds.test.ts	21 schema + integration tests

Endpoints

POST  /api/v1/3ds/initiate      – begin a 3DS session (returns ACS URL / CReq / PAReq)
POST  /api/v1/3ds/verify        – submit challenge result (transStatus / PARes)
GET   /api/v1/3ds/status/:id    – poll session state
POST  /api/v1/3ds/cancel        – cancel an active session

Spec versions supported

• 3DS 1.0.2 – PAReq/PARes redirect flow
• EMV 3DS 2.1 / 2.2 – frictionless + challenge flows (CReq/CRes)

Tests

45 tests, all passing:

Test Files  2 passed (2)
     Tests  45 passed (45)

Closes #925

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