fix: coerce Encryptor::decrypt() to string

[justlevine]

What

Ensures Settings::get_api_key() returns a string even if decryption fails - e.g. from a legacy key.

Why

Fixes a regression in #71 where the string coersion wasn't appl

Related Issue(s):

How

Testing Instructions

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Additional Info

Checklist

• I have read the Contribution Guidelines.
• I have read the Development Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint etc.).
• My code has detailed inline documentation.
• I have updated the project documentation as needed.

[Copilot]

Pull request overview

Ensures Settings::get_api_key() always returns a string even when decryption fails (e.g., legacy/invalid stored keys), avoiding type errors under strict_types.

Changes:

• Coerce the decrypted API key result to a string-safe return value in Settings::get_api_key().
• Simplify Settings::regenerate_api_key() to return the generated key directly.

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