feat: add balancer database cluster manifest

[TineoC]

This PR updates the balancer deployment to use the shared, multi-tenant codeforphilly-cluster PostgreSQL instance.

Changes:

• New Sandbox Overlay: Created deploy/manifests/balancer/overlays/sandbox.
• ConfigMap for Database Settings: Added configmap.yaml in the sandbox overlay to store SQL_HOST, SQL_PORT, etc.
• Shared DB Connection: The deployment is patched to use envFrom pointing to the new balancer-db-config ConfigMap.
• Credentials: The application uses the balancer-db-credentials secret (which must be created in the cloudnative-pg namespace) to source the SQL_PASSWORD.

DEPENDS ON: CodeForPhilly/cfp-sandbox-cluster#135 (which defines the shared cluster and the balancer database).

[TineoC]

Superseded by #507 which integrates Gateway API, CORS, and CNPG changes into a single branch.