Returns the trainee's mean daily step count over a trailing 7 calendar-day window (the end day and the six days before it, both ends inclusive). The window ends on today by default, or on the optional date parameter when supplied — pass it to inspect an earlier week. Days on which no step count was recorded are excluded from the average — they are NOT treated as zero — so daysCounted reports how many of the 7 days actually contributed. The figure is rounded to the nearest whole step. When no day in the window has a recorded step count, the response is 200 with averageSteps: null and a human-readable message.