Stickers

Per-room uploaded stickers: a large image sent as a message, picked from a tray beside the emoji picker, the way Discord and Slack both do it.

Why deferred

• A sticker is not a glyph. It has no reaction path, no : autocomplete, and no inline render inside text, so it shares almost nothing with custom emoji beyond the upload — it is a new message type, with its own render component, its own per-item weight in the message list, and its own picker surface.
• The emoji work answers the storage and permission questions first (a room-owned blob prefix, a count cap, a manage permission), so building stickers before it means answering them twice.

Revisit when: rooms want a bigger, standalone image than an inline glyph — at which point the sticker set reuses the emoji table's shape, cap and permission, and the only new work is the message type.