Uniqueness is not enforced when writing concurrently

[xixixao]

This pattern of multi-upsert violates uniqueness constraints on the docs:

async function _saveAll(
  ctx: MutationCtx,
  perms: WithoutSystemFields<Doc<"perms">>[],
) {
  return await asyncMap(perms, async (perm) => {
    const existing = await ctx.table("perms").get("id", perm.id);
    if (existing === null) {
      return await ctx.table("perms").insert(perm);
    } else {
      return await existing.replace(perm);
    }
  });
}

This is because the reads happen in concurrent, and then the writes happen. This is not really specific to Ents, but Ents should catch this footgun.