Previously when PG_REUSE=1 and PG_VERSION existed but the target database had
never been successfully created (e.g. devshell exited mid-init in a prior run),
postgres-init skipped createdb and the subsequent psql connection failed with
'database "<db>" does not exist'.
Now on reuse path we probe pg_database and create the target DB if missing,
making postgres-init fully idempotent across stale-state recovery.
Adds postgres-init-reuse-missing-db test.
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.
