refactor(logging): replace structlog with stdlib logging compat layer

- Remove structlog dependency from backend/pyproject.toml
- Add app.utils.logging_compat shim for keyword-arg logging API
- Add app.utils.json_formatter for JSON log output with extra fields
- Update all backend modules to use logging_compat.get_logger()
- Update docstrings in log_sanitizer.py and json_formatter.py
- Update test comment in test_async_utils.py
- Record 406 failing tests in Docs/Tasks.md for tracking

backend/app/routers/setup.py

@@ -7,7 +7,7 @@ return ``409 Conflict``.
 
 from __future__ import annotations
 
-import structlog
+from app.utils.logging_compat import get_logger
 from fastapi import APIRouter, status
 
 from app.dependencies import AppDep, SettingsDep, SettingsServiceContextDep
@@ -17,7 +17,7 @@ from app.services import setup_service
 from app.utils.runtime_state import update_app_settings
 from app.utils.setup_state import is_setup_complete_cached, set_setup_complete_cache
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 router = APIRouter(prefix="/api/v1/setup", tags=["setup"])