Report comparison counts per handle [first-party]

For the first-party client that records side-by-side comparison verdicts; any other token is refused with 403. The body is a full snapshot taken at reported_at: each handle is set to its totals and a handle left out is removed, so sending the same snapshot again changes nothing and an older one never overwrites a newer one. The totals reach stats.comparisons and stats.wins on the next hourly recount. Answers 503 not_migrated until the platform's table is in place.