T412401: Plan and Implement API Versioning
Changes: Introduced versioned routers (v1 and v2) mirroring existing endpoints. Applied deprecation middleware to v1 and legacy root routes, signaling deprecation (Deprecation: true) and sunset date (2026-03-31). Updated server.js to mount /api/v1 and /api/v2, with root / aliasing to v1 for backward compatibility. Added deprecation.ts middleware to inject RFC 8594 headers. Documented versioning and deprecation in CHANGELOG.md. Updated README.md with API Versioning and Migration Guide. Added docs/API_VERSIONING.md detailing strategy, structure, and developer workflow.
Bug: T412401
Closes T412401