add weight delta to stat

internal/db/db.go

@@ -239,16 +239,27 @@ func (db *DB) GetStatBetween(chatID int64, startTime time.Time, endTime time.Tim
 	if err != nil {
 		return nil, err
 	}
+	firstWeight := 0.0
+	lastWeight := 0.0
+	countWeights := 0
 	for weightCursor.Next(context.Background()) {
 		var result Weight
 		if err := weightCursor.Decode(&result); err != nil {
 			log.Fatal(err)
 		}
 		res["Вес кг"] = result.Weight
+		if firstWeight == 0 {
+			firstWeight = result.Weight
+		}
+		lastWeight = result.Weight
+		countWeights++
 	}
 	if err := weightCursor.Err(); err != nil {
 		log.Fatal(err)
 	}
+	if countWeights > 0 {
+		res["Изменение веса"] = lastWeight - firstWeight
+	}
 
 	return res, err
 }