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/pyproject.toml

@@ -15,7 +15,6 @@ dependencies = [
     "aiosqlite>=0.20.0",
     "aiohttp>=3.11.0",
     "apscheduler>=3.10,<4.0",
-    "structlog>=24.4.0",
     "bcrypt>=4.2.0",
     "geoip2>=4.8.0",
     "prometheus-client>=0.21.0",