The task In our internal business system, I built a feature that stamps a company seal (the square kaku-in used on Japanese business documents) onto invoices, quotes, and purchase orders. The documents are Excel templates filled with data and converted to PDF via LibreOffice. The requirements: Never distribute the seal image by itself — no public URL, no image-serving API. It may only be seen stamped on a document Per-document ON/OFF toggle Once a document is finalized, replacing ...