Backend: Add `stats` JSON object to `BackendResponse`

[leriomaggio]

Continuing in this issue the conversation started in the latest PR

(@OliverDavis comment)

How about this as a return object for both the annotate and forget endpoints?

{ "data": [current return object], "stats": {name, value for model fit stats} }

RE: I think that makes lots of sense. Also, appending the new stats objects to the current backend response is super quick to do too.

I think we should just decide what the stats object should contain, and which metric we want to calculate.

One first hypothesis can be to generate a per-class score (e.g. per-class precision), along with an overall score for all the faces currently on the board.

In other words, the stats object can have the following structure:

{
  stats: { 
      happy: happy score in [0, 1]
      sad: sad score  in [0, 1]
      angry: angry score  in [0, 1]
      disgust: disgust score  in [0, 1]
      fear: fear score  in [0, 1]
      surprise: surprise score  in [0, 1]
      overall: overall global score  in [0, 1]
}

When a new face will come in, updated scores for old 24 + 1 faces on the board will be returned.

Would that make any sense?