fix(documents): gate selected_custom_fields behind editable guard; bound index cap to column width

Addresses review: add selected_custom_fields to offers/issued-orders
NON_STATUS_UPDATE_FIELDS so a non-editable document rejects selection
edits (status-only transitions still pass); tighten Zod cap 200->50 to
stay within VARCHAR(255).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/schemas/issued-orders.schema.ts

@@ -45,7 +45,7 @@ export const CreateIssuedOrderSchema = z.object({
   // PDF. Omitted/empty ⇒ none. Update schema derives via .partial().
   selected_custom_fields: z
     .array(z.number().int().nonnegative())
-    .max(200)
+    .max(50)
     .optional(),
 });