Auto-generating and persisting the key to data/.encryption_key co-locates
the key with the database, defeating encryption at rest if an attacker can
read the data directory. It also silently loses all encrypted secrets if the
data volume is recreated.
Replace the auto-generation fallback with a hard startup error that tells
operators exactly what to do:
- Upgraders from the JWT_SECRET-derived encryption era: set ENCRYPTION_KEY
to their old JWT_SECRET so existing ciphertext remains readable.
- Fresh installs: generate a key with `openssl rand -hex 32`.
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