ci(postgres): teach the PG-compat tooling the audit 6-9 divergence classes

The whole-repo MariaDB<->PostgreSQL audits surfaced classes the checker and
review guide did not yet cover. Add them:

Static checker (.github/helper/postgres_compat.py) - new mechanical breaks:
- .rlike() / raw RLIKE: frappe rewrites REGEXP->~* on Postgres but NOT RLIKE.
- Cast(x, "char") / raw CAST AS CHAR: bare CHAR is character(1) on Postgres
  and truncates multi-digit values; use "varchar".
(Both flag zero production code; the only repo hit is in patches/, which the
hook already excludes.)

Greptile config + POSTGRES_COMPATIBILITY.md - new semantic/hard classes:
- aggregate (Sum/Count) selected next to bare columns with no GROUP BY at all.
- .like()/LIKE on a non-text column (bigint ILIKE) -> Cast_ to varchar.
- get_all(fields=["CapitalCase"]) identifier-case (extends the get_value case).
- bool into a Check column via qb.update().set() (extends set_value/db_set).
- int/int division: float a literal (col/1440 -> col/1440.0).
- Concat over a nullable column leaking a bare prefix on Postgres.
- clarify REGEXP/.regexp() is translated but RLIKE/.rlike() is not.

.github/helper/postgres_compat.py

@@ -62,6 +62,10 @@ SQL_PATTERNS: list[tuple[re.Pattern, str]] = [
 	 "single-quoted column alias breaks on Postgres -> use a bare or double-quoted alias"),
 	(re.compile(r"\bif\s*\(", re.I),
 	 "SQL IF() is MySQL-only -> use CASE WHEN ... THEN ... ELSE ... END (frappe.qb.Case())"),
+	(re.compile(r"\brlike\b", re.I),
+	 "RLIKE is MySQL-only -> frappe rewrites REGEXP->~* on Postgres but NOT RLIKE; use REGEXP / .regexp() / ~"),
+	(re.compile(r"\bcast\s*\([^)]*\bas\s+char\b", re.I),
+	 "CAST(... AS CHAR) is character(1) on Postgres and truncates -> CAST AS VARCHAR (frappe Cast_(x, 'varchar'))"),
 ]
 
 # UPDATE ... JOIN: both keywords in the same SQL string.
@@ -71,6 +75,10 @@ MYSQL_RESULT_KEYS = {"Column_name", "Key_name", "Seq_in_index", "Non_unique", "I
 
 SET_BOOL_FUNCS = {"set_value", "db_set"}
 
+# query-builder cast helpers: pypika Cast / frappe Cast_. A "char" target type is character(1)
+# on Postgres (truncates); "varchar" is the full-length cast.
+CAST_FUNCS = {"Cast", "Cast_"}
+
 # frappe.get_all / get_list: frappe's db_query SILENTLY drops ORDER BY for `distinct` queries on
 # Postgres (the ORDER BY column must appear in the SELECT-DISTINCT list), so `distinct=True` together
 # with a literal `order_by` is a no-op on PG and the result comes back unordered.
@@ -178,6 +186,18 @@ class Visitor(ast.NodeVisitor):
 			if has_distinct and has_literal_order:
 				self._flag(node, f"{name}(distinct=True, order_by=...) -> frappe drops ORDER BY for distinct queries on Postgres; sort in python instead, e.g. sorted(..., key=str.casefold)")
 
+		# query-builder .rlike(...): pypika emits the MySQL-only RLIKE operator, which frappe does
+		# NOT translate for Postgres (it rewrites only REGEXP -> ~*).
+		if name == "rlike":
+			self._flag(node, ".rlike() emits MySQL-only RLIKE (not translated on Postgres) -> use .regexp() (rewritten to ~*) or .like()")
+
+		# Cast(col, "char") / Cast_(col, "char"): on Postgres a bare CHAR is character(1) and truncates
+		# (e.g. CAST(12 AS CHAR) -> '1'); use "varchar" for a full-length string cast.
+		if name in CAST_FUNCS:
+			for arg in (*node.args, *(kw.value for kw in node.keywords)):
+				if isinstance(arg, ast.Constant) and isinstance(arg.value, str) and arg.value.strip().lower() == "char":
+					self._flag(node, f"{name}(..., 'char') is character(1) on Postgres and truncates -> use 'varchar'")
+
 		self.generic_visit(node)
 
 	def visit_Subscript(self, node: ast.Subscript) -> None: