Returns the trainee's recorded body weight over the given date range — one entry per day that has a logged weight, in ascending date order. Days without a recorded weight are omitted (the list is not padded). Both from and to are inclusive. The range may not exceed 1 year (366 days); pull longer histories with successive ranged requests. When no weight was recorded in the range, the response is 200 with entries: [] and a human-readable message.