Submit map scores across multiple matches in one call

Submits map scores for up to 50 (match, map) items in a single request. Each item is independently scope-checked against the key's bound game; an off-scope or otherwise failing item is rejected in its per-item envelope (status "failed", error = bare error_* code) WITHOUT failing the whole request. The response is ALWAYS HTTP 200 - inspect per-item status. Requires the game.scores:write permission. Score submission is naturally idempotent: an unconfirmed map is overwritten on re-submit and an already-confirmed map returns a per-item error_score_already_submitted_by_game, so a retried batch is safe without an Idempotency-Key.