Set expiration from original V3 creation date

[mkysel]

No description provided.

[macroscopeapp]

Compute originator envelope expiry from original creation timestamps and add created_at to key package reads in reader.go and models

Expiry in migrator.Transformer.buildAndSignOriginatorEnvelope now uses the payload’s creationTime; KeyPackage gains CreatedAt populated by the reader and tests update to write created_at.

📍Where to Start

Start with migrator.Transformer.buildAndSignOriginatorEnvelope and its new creationTime parameter in transformer.go.

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Macroscope summarized 029f4b3.