Validate that RHDH works correctly when customers configure pluginDivisionMode: schema in their app-config.yaml. This is a Backstage backend feature that should work OOB - we need to test and officially support it.

Acceptance Criteria:
[ ] Deploy RHDH with external PostgreSQL and pluginDivisionMode: schema configured
[ ] Verify core plugins (catalog, scaffolder, auth, permission, search) create their schemas with rhdh_ prefix
[ ] Verify Knex migrations run successfully in each schema
[ ] Verify no connection attempts to postgres maintenance database
[ ] Verify data isolation between plugin schemas
[ ] Test with a DB user that has only CREATE SCHEMA permissions (no CREATEDB)
[ ] Document the configuration in external-db.md