docs: document LOG_LEVEL as a backend env variable (#258)

[olaayoade27-coder]

pr close #258

Add LOG_LEVEL to the backend environment variable table in README.md, .env.example, and docs/configuration.md. The Winston-based structured JSON logging with request IDs was already implemented; this commit closes the documentation gap so operators know the variable is configurable for the backend (error|warn|info|http|debug).

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• Bug fix (non-breaking change which fixes an issue)
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• Breaking change (fix or feature that would cause existing functionality to change)
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