feat: expose row-level can_edit/can_delete on select *

Compute per-row editability and deletability from a table's row-level
security policies and return them as synthetic columns so clients can
hide edit/delete affordances for rows the user cannot change.

- Introspect pg_policies and relrowsecurity at schema-cache load and
  combine the UPDATE/DELETE USING qualifiers per table (permissive OR,
  restrictive AND).
- Store the combined qualifiers on Table and inject can_edit/can_delete
  as computed select fields when expanding `select *`, only for
  RLS-enabled tables with a matching policy (COALESCE'd to a boolean).
- Keep the computed columns out of the OpenAPI spec so they are not
  rendered as regular fields.
- Add a cfExpression field to CoercibleField to carry raw SQL
  expressions through the planner to SqlFragment.

src/library/PostgREST/Query/SqlFragment.hs

@@ -249,6 +249,7 @@ pgFmtField table cf = case cfToTsVector cf of
   _                      -> fmtFld
   where
     fmtFld = case cf of
+      CoercibleField{cfExpression=Just expr}                                  -> SQL.sql (encodeUtf8 expr)
       CoercibleField{cfFullRow=True}                                          -> pgFmtIdent (qiName table)
       CoercibleField{cfName=fn, cfJsonPath=[]}                                -> pgFmtColumn table fn
       CoercibleField{cfName=fn, cfToJson=doToJson, cfJsonPath=jp} | doToJson  -> "to_jsonb(" <> pgFmtColumn table fn <> ")" <> pgFmtJsonPath jp