fix(config): unify log_level from config.yaml across Gateway and debug entry points (#2601)

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>

backend/app/gateway/app.py

@@ -29,11 +29,12 @@ from app.gateway.routers import (
     uploads,
 )
 from deerflow.config import app_config as deerflow_app_config
+from deerflow.config.app_config import apply_logging_level
 
 AppConfig = deerflow_app_config.AppConfig
 get_app_config = deerflow_app_config.get_app_config
 
-# Configure logging
+# Default logging; lifespan overrides from config.yaml log_level.
 logging.basicConfig(
     level=logging.INFO,
     format="%(asctime)s - %(name)s - %(levelname)s - %(message)s",
@@ -164,6 +165,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     # Load config and check necessary environment variables at startup
     try:
         app.state.config = get_app_config()
+        apply_logging_level(app.state.config.log_level)
         logger.info("Configuration loaded successfully")
     except Exception as e:
         error_msg = f"Failed to load configuration during gateway startup: {e}"