Update all documentation to reflect new global LOG_LEVEL environment variable.
- Add LOG_LEVEL to configuration reference tables
- Update deployment guides with LOG_LEVEL=error examples
- Clarify dev (info) vs production (error) log level recommendations
- Add LOG_LEVEL to all .env examples and quick start guides
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Align project docs with current behavior using progressive disclosure and bilingual deep-dive guides. Add per-page admin screenshots with consistent page-* naming to make UI documentation clearer.