refactor(reports): label revenue categories from a key column

CATEGORY_LABEL_EXPR wraps the classifying CASE, so it only works where the
classification happens in the same SELECT. A report that classifies in a
subquery and labels in the wrapper has a plain key column to label instead.

CATEGORY_LABEL_OF takes that key expression; CATEGORY_LABEL_EXPR is now
defined through it, so its three existing callers are unchanged. Mirrors
CATEGORY_LABEL_OF in operations-classification.ts.

Claude-Session: https://claude.ai/code/session_01LoY3hNWqcaAC1pYmGPN7jr

apps/edr-freight-api/src/modules/reports/revenue-classification.ts

@@ -139,8 +139,15 @@ const labelCase = (expr: string, options: ReportFilterOption[]): string =>
     .map((o) => `WHEN '${o.value}' THEN '${o.label.replace(/'/g, "''")}'`)
     .join('\n  ')}\nEND`;
 
+/**
+ * The same labelling applied to a key that is already a column — for reports
+ * that classify in a subquery and label in the wrapper.
+ */
+export const CATEGORY_LABEL_OF = (keyExpr: string): string =>
+  labelCase(keyExpr, REVENUE_CATEGORIES);
+
 /** The category as a business label rather than its key, for display columns. */
-export const CATEGORY_LABEL_EXPR = labelCase(REVENUE_CATEGORY_EXPR, REVENUE_CATEGORIES);
+export const CATEGORY_LABEL_EXPR = CATEGORY_LABEL_OF(REVENUE_CATEGORY_EXPR);
 
 /**
  * Period-over-period change, as a percentage.