This reads like ChatGPT, unfortunately. 80% of what is described is a stock web app, and the decentralised part is mostly hand-waving with no clear explanation

This reads like ChatGPT, unfortunately. 80% of what is described is a stock web app, and the decentralised part is mostly hand-waving with no clear explanation of what the boundaries are between a API and courier server. Any existing ActivityPub implementation already matches whatever is described.