Comment threads on task cards

create table comments (
  id          uuid primary key default gen_random_uuid(),
  task_id     uuid not null references tasks(id),
  parent_id   uuid references comments(id),   -- one level only,
                                                -- enforced in API
  author_id   uuid not null references users(id),
  body        text not null,
  created_at  timestamptz not null default now(),
  deleted_at  timestamptz                       -- soft delete
);

create table comment_reads (
  task_id    uuid not null references tasks(id),
  user_id    uuid not null references users(id),
  read_up_to timestamptz not null,
  primary key (task_id, user_id)
);

create index comments_task_created
  on comments (task_id, created_at);

export function useAddComment(taskId: string) {
  const qc = useQueryClient();
  return trpc.comments.create.useMutation({
    onMutate: async (input) => {
      const temp = { ...input, id: `temp-${nanoid()}`,
                     createdAt: new Date(), pending: true };
      qc.setQueryData(key(taskId), (prev) =>
        [...(prev ?? []), temp]);
      return { tempId: temp.id };
    },
    onSuccess: (row, _v, ctx) => {
      // reconcile temp id to real id so the
      // realtime append does not duplicate it
      qc.setQueryData(key(taskId), (prev) =>
        prev.map((c) => c.id === ctx.tempId ? row : c));
    },
    onError: (_e, _v, ctx) => {
      qc.setQueryData(key(taskId), (prev) =>
        prev.filter((c) => c.id !== ctx.tempId));
    },
  });
}