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/services/settings_service.py

@@ -8,14 +8,14 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.repositories import settings_repo
 
 if TYPE_CHECKING:  # pragma: no cover
     import aiosqlite
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 _KEY_MAP_COLOR_THRESHOLD_HIGH = "map_color_threshold_high"
 _KEY_MAP_COLOR_THRESHOLD_MEDIUM = "map_color_threshold_medium"