feat(backend): Add API versioning middleware to NestJS routes

[Viky207]

Problem

Backend routes lack versioning, making breaking changes risky.

Task

Implement API versioning middleware for NestJS supporting Accept-Version header and URL path versioning.

Acceptance Criteria

• Middleware supports Accept-Version: application/vnd.quickex.v1+json
• URL path versioning at /v1/...
• Default version v1 for unversioned requests
• 415 for unknown versions
• All controllers migrated to versioned routes
• Swagger docs reflect versioned paths
• Integration tests cover negotiation and fallback