Honor APP_DEBUG to toggle Slim error detail output

[Kvnbbg]

Motivation

• Prevent accidental leakage of exception details in production by making the Slim error middleware respect the APP_DEBUG environment variable.

Description

• Read APP_DEBUG via getenv and normalize it with filter_var(..., FILTER_VALIDATE_BOOLEAN) into $displayErrorDetails.
• Pass $displayErrorDetails to $app->addErrorMiddleware so error details are shown only when APP_DEBUG is true and update the related comment.

Testing

• No automated tests were run for this lightweight change.

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Codex Task

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