Allow using an external PostgreSQL database instead of bundled StatefulSet

Scope:

• Add database.deploy flag (default true); when false, skip StatefulSet, Service, init ConfigMap

• Fix the waitForDb init container to target database.server.host (currently hardcoded to internal service name even when host != "internal")

• Document required database setup: create databases (costonprem_ros, costonprem_kruize, costonprem_koku), users, grants, and the expected secret format

• Update install-helm-chart.sh to skip DB credential generation when database.deploy: false

Estimate: Medium-High (most template changes, init container fixes, documentation)