Postgres audit_ledger append-only guard gains exactly one
execution entry: cloud/ledger_pg_guard.apply_pg_guard, runnable
as 'python3 -m cloud.ledger_pg_guard' (explicit ops command,
not a migration script). It executes PG_GUARD_SQL verbatim from
the constant -- no second copy of the SQL anywhere -- refuses
non-PgConnection targets, does not commit itself (caller/CLI
commits), and refuses with exit 2 on a non-postgres backend so
a no-PG environment can never claim hardening. ensure_schema
still skips PG triggers; POSTGRES_SCHEMA still builds only the
table. Tests lock all of this with a recording fake connection
(no PG execution is faked), a source scan asserting only
ledger.py and ledger_pg_guard.py reference the constant, and
CLI dry-run/refusal cases. Real-PG syntax and UPDATE/DELETE
blocking verification is explicitly left to B5, not claimed here.
(cherry picked from commit b1f6c9158d628186965a9e57576ff8c9500e3f73)