Configure CORS Allowed Origins via ConfigMaps

[TineoC]

Addresses #497

This PR makes CORS allowed origins configurable via environment variables and Kubernetes ConfigMaps.

Changes:

• Modified server/balancer_backend/settings.py to read CORS_ALLOWED_ORIGINS from environment, defaulting to localhost:3000.
• Updated base deployment manifest to include envFrom from a ConfigMap named balancer-config.
• Added an empty configMapGenerator to the base kustomization.yaml.
• Created sandbox and production overlays with their respective CORS_ALLOWED_ORIGINS configured in a configmap.yaml patch.

Deployment note: Ensure that the balancer-config ConfigMap is created or generated in the target namespace.