Assemble the Postgres DSN from DATABASE_ENGINE/HOST/PORT/NAME/USER/
PASSWORD/OPTIONS so only the password needs to live in a secret; the
rest can go in a ConfigMap. DATABASE_URL remains a full-DSN override
(used by the sqlite test suite). Credentials are percent-encoded, the
port and options are omitted when empty, and the k8s migrate
initContainer now also reads the config ConfigMap.
