Fix TOTP key invalidation by persisting encryption key

[google-labs-jules]

Modified backend/models/user.py to prioritize TOTP_ENCRYPTION_KEY env var, fall back to Redis, and persist generated keys to Redis. This prevents key invalidation on Azure deployments where the local filesystem is ephemeral.

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PR created automatically by Jules for task 10011267980899559604 started by @8JP8

[google-labs-jules]

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