refactor(proofs): run OCR in post_save signal instead of create

[raphodn]

What

Following #543

Some refactoring :

• call the OCR async task in the model's post_save signal (instead of the API view)
• don't run the task when running the tests (locally or in the CI) : avoids calls to Google API
• rename run_ocr_task to fetch_and_save_ocr_data for clarification

--> similar to what we have currently with the Location & Product async tasks