Add VERBOSE_ERRORS env var to surface internal error messages

[clementfavre]

Why

issue #5501
Right now whenever something fails on the backend (a Let's Encrypt request, an nginx reload, an openssl call, etc.) the UI just shows Internal Error and the real cause is buried in the container logs => you have to docker logs npm every time a cert won't issue

This adds an opt-in env var VERBOSE_ERRORS=true that returns the real error message in the API response (and therefore in the UI toast), so users can see what actually went wrong without leaving the browser

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• API changes
• Performance improvement
• Test addition or update

AI Usage

• AI was used to write this
• AI was used to review this