Design an ephemeral "stories" service where a user posts short photo or video clips that are visible to their followers for 24 hours and then automatically stop being served.

Operating context. Hundreds of millions of people open the app daily. Each posts a few stories and watches many. Media clips are a few seconds long, up to about 10 MB, and reads dwarf writes. When a viewer opens the app they see a "tray" of followed authors who currently have fresh, unseen stories, tap in, and watch them in order. The system tracks which viewer saw which story and gives the author an aggregate viewer list. Twenty-four hours after posting, a story must disappear from every tray and no longer be playable.

Out of scope. The video transcoding pipeline internals, DRM, direct-message replies to stories, ad insertion, and recommending whose story to watch. Assume other teams own those.

What to produce. A high-level architecture covering: the upload path and where media bytes vs metadata live, storage and CDN delivery for playback, the read model that assembles a per-viewer tray of authors with unexpired unseen stories, the expiry-and-reclamation mechanism, and how per-viewer "seen" state and author-facing view counts are recorded off the hot path. Sketch the components and the request flow; checkpoints will probe the tray read model and the expiry design.