Config endpoint returns only orgs that correspond to allowed corpora

[annaCPR]

Description

• require an app_token header for the /config endpoint
• validate and decode the app_token in the /config endpoint
• only return stats for organisations linked to allowed corpora from the app_token

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Reviewer Checklist

• DB_CLIENT DEPENDENCY IS ON THE LATEST VERSION
• The PR represents a single feature (small driveby fixes are also ok)
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[linear]

PDCT-1502 Update backend config to only show corpora in config token

[katybaulch]

Great work! Minor comment based on context that I might not have made clear which is blocking - but happy as soon as that is resolved for this to go in 🍪

[katybaulch]

All good once the commented out code has been removed!