[Users] Add Create User Route (POST /users)

[nourshoreibah]

User Story

As a user I want to create a new user in the Users table.

Description

• Add the POST /users route to the existing users Lambda using:
  node tools/lambda-cli.js add-route users POST /users --body name:string,isAdmin:boolean --status 201
• If the users Lambda does not exist, create it first with:
  node tools/lambda-cli.js init-handler users
• Implement insert logic using Kysely for type-safe SQL.
• Test using Swagger UI and create Jest unit tests for success and validation failures.

Acceptance Criteria

• Route successfully creates a user record in the local database.
• Swagger returns 201 with created user data.
• Jest tests pass for valid and invalid input payloads.