By: Michael Lynch <git@mtlynch.io>
Show emoji reaction picker on media pages (m7-1)
Add the reaction-picker UI to each media page: the five allowed emoji render
as buttons below the image, highlighting the viewer's current reaction, and
imported reactions continue to display with their author and time.
The POST /m/{id}/reactions endpoint intentionally returns HTTP 501 until the
next stacked commit persists reactions.
To support the picker, ReadReactions now returns each reaction's author ID and
parent media ID, and a new AllowedReactionEmoji accessor exposes the emoji list
to the handler.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>