Centralize log level parsing in `logging_level_from_config()` and
application in `apply_logging_level()` within `deerflow.config.app_config`.
- Gateway lifespan applies configured log level on startup
- `debug.py` uses shared helpers instead of local duplicates
- `apply_logging_level()` targets only `deerflow`/`app` logger hierarchies
so third-party library verbosity is not affected; root handler levels
are only lowered (never raised) to allow configured loggers through
without suppressing third-party output; root logger level is not modified
- Config field description updated to clarify scope
- Tests save/restore global logging state to avoid test pollution
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
