FE sends ?sortBy=publishedAt&order=desc on /listings and was getting 400
"property order should not exist". Add optional order ('asc'|'desc') to
the DTO, plumb through query/handler/cache key, and apply direction in
the Prisma orderBy. priceAsc/priceDesc still encode their own default
direction but honour an explicit order override.
Co-Authored-By: Paperclip <noreply@paperclip.ing>