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/utils/config_parser.py

@@ -28,12 +28,12 @@ import configparser
 import re
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 if TYPE_CHECKING:
     from pathlib import Path
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Compiled pattern that matches fail2ban-style %(variable_name)s references.
 _INTERPOLATE_RE: re.Pattern[str] = re.compile(r"%\((\w+)\)s")