ci(postgres): flag division by a possibly-zero divisor in the compat guard (#56363)

Adds the division-by-zero divergence class to the PG-compat review tooling:
on a divisor that the data can drive to 0 (e.g. Sum(a)/Sum(b)), MariaDB
returns NULL for division by zero while PostgreSQL raises `division by zero`
and aborts the query. The portable fix is to wrap the divisor in
NullIf(divisor, 0), which yields NULL on both engines (matching MariaDB).

- .greptile/config.json: add it to the "would ERROR on PostgreSQL" list.
- .github/POSTGRES_COMPATIBILITY.md: document it under §1 (hard breaks).
- .github/helper/postgres_compat.py: note it in the docstring as a
  deliberately-not-statically-checked semantic divergence (data-dependent,
  like integer-division intent), so it stays a reviewer/Greptile concern.

Tooling-only; no source query changes. The instance fix shipped in #56361.

.github/helper/postgres_compat.py

@@ -7,8 +7,8 @@ that static analysis can catch reliably with a low false-positive rate.
 
 It deliberately does NOT try to catch the *semantic* divergences (loose GROUP BY,
 case-sensitive ==/IN, NULL ordering, ORDER BY ... LIMIT 1 tiebreakers, integer-division
-intent, savepoint discipline) — those genuinely need the test suite. Run the full suite
-on a Postgres site for those.
+intent, division by a possibly-zero divisor, savepoint discipline) — those genuinely need
+the test suite or a human/Greptile reviewer. Run the full suite on a Postgres site for those.
 
 Escape hatch: put `# pg-ok` anywhere on the offending statement's line span (e.g. on a
 `SHOW INDEX` query that lives inside an `if frappe.db.db_type == "mariadb":` branch).