Harden encryption failure handling

[Rhutvik-pachghare1999]

What changed

• Corrected the encryption module terminology to describe Fernet accurately.
• Invalid configured keys now fail closed instead of silently generating a replacement key.
• Tampered ciphertext and non-numeric decrypted sensor values now raise errors instead of becoming 0.0.
• Key rotation refuses to overwrite an existing backup and restores the previous key if persistence fails.
• Added focused regression tests for invalid keys, ciphertext tampering, and invalid sensor plaintext.

Why

The previous behavior could turn a cryptographic failure or corrupted telemetry value into a valid-looking zero. In an industrial prediction pipeline, that hides integrity failures and can influence downstream model decisions.

Validation

The added tests are designed to run with the existing pytest suite. CI should verify compatibility with the repository's current dependencies and Python versions.

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