ref: visitor endpoint, modified render-deploy.yaml

[devcavin]

This pull request introduces several important changes to the visitor registration flow in the backend, primarily making the zoneId field mandatory for visitor registration and updating related service logic. Additionally, there is a minor update to the deployment workflow file.

Visitor Registration Changes:

• Made the zoneId field in RegisterVisitorRequest a required (non-nullable) property, enforcing that all visitor registrations must specify a zone. (backend/src/main/kotlin/io/github/devcavin/backend/web/dto/visitor/VisitorDTOs.kt)
• Updated VisitorService to remove the null check for zoneId, reflecting its new required status. (backend/src/main/kotlin/io/github/devcavin/backend/service/VisitorService.kt)

DevOps / Workflow:

• Added a blank line to the top of .github/workflows/deploy-render.yaml for formatting consistency. (.github/workflows/deploy-render.yaml)