Wrap db-tool, postgres-init, postgres-cleanup with lib.makeOverridable so
consumers can inject extension-enabled PostgreSQL via .override { postgresql = ...; }.
Add PG_CONF_FILE: when set, replaces script-generated postgresql.conf entirely
(runtime port and unix_socket_directories still appended/overridden).
Previous basename guard only ran the function when invoked as the
postgres-init binary, but consumer shellHooks source the script to
inherit POSTGRESQL_HOST/PGURL/etc. Sourcing left the function defined
but never called, leaving WIPE_PGURL/PGURL unset and breaking devshell
entry in proxydoe and ruststats.
