Bug Description

When an artist is not present in the database, the "Dashboard" and "Top Sessions" pages fail to load properly. Additionally, the "Top Sessions" page sends repeated requests to the self-hosted instance without attempting to retrieve the missing data from Spotify or update the database accordingly.

Steps to Reproduce

1. Navigate to the "Dashboard" or "Top Sessions" pages.
2. Ensure the artist in question does not exist in the database.
3. Observe the pages failing to load and the repeated requests being made to the instance.

Expected Behavior

When an artist is missing from the database, the backend should:

1. Detect the missing data.
2. Automatically re-fetch the artist information from Spotify.
3. Update the database with the new data to prevent further failures.