optimize statistics

[immone]

The statistic features in the application might end up having issues when the number of articles $n$ grows large, slowing down the article statistics views. Here are some possible things to consider to speed this up:

• The domain and time series statistics are currently set to one API endpoint. Refactor this into 3 separate endpoints in server/src/routes.py and the corresponding DB functions in server/src/views/data_analysis/stats_analyzer.py.

• Implement some level of cache to not fetch statistics if the articles table has not been updated.

• The mapping from all news to individual word appearances is currently fully done in front end, and is $\mathcal O(n)$ with similar number of access times to the frequency data structure. It might be faster to use, e.g., Python's pandas.Series.value_counts in back end and then just send the json over.