[FE] Display the amount saved when a transcription is retrieved from cache

[mshkkkkk]

Description
When an existing transcription is retrieved from the shared cache, the model is not called again and the processing cost is zero.
As discussed with the Product Owner, the interface could inform the user how much they saved because the cached transcription was reused. This would make the benefit of the cache visible and explain why the page was processed without increasing the document cost.
Example message:
You’ve saved $X thanks to our premium cache feature.

The transcription cache is shared across user accounts, but the notification must not expose whether or by whom the same document was processed previously.

Acceptance Criteria

• - When a transcription is retrieved from cache, the user can see the amount saved.
• - The displayed amount represents the estimated cost of the avoided model call.
• - The message does not reveal information about other users or their documents.
• - No savings message is displayed when the model is called normally.