Toggle Reaction

Toggle a reaction on a post or comment.

Exactly one of post_id or comment_id must be supplied — both empty or both set both raise 400 INVALID_INPUT. The emoji slug must be one of ALLOWED_REACTIONS (defined in app/models/reaction.py); arbitrary unicode emoji are rejected so the reactions UI stays curated.

Idempotent toggle semantics: if the (user, target, emoji) row already exists it's deleted (reaction removed). Otherwise it's created and a post_reaction notification fires for the content author + a reaction.added webhook event fans out. Removals never fire notifications.

Side effects on add: post-detail HTML cache invalidated for the affected post id (so the reactions bar redraws on the next visitor). For comment reactions the parent post id is used.

Rate-limited 120 toggles per minute per user under the reaction bucket. Returns the full ReactionSummary for the target so the client can re-render without a follow-up GET.