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/startup.py

@@ -26,7 +26,7 @@ from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
 import aiohttp
-import structlog
+from app.utils.logging_compat import get_logger
 from apscheduler.schedulers.asyncio import AsyncIOScheduler  # type: ignore[import-untyped]
 
 from app.db import init_db, open_db
@@ -59,7 +59,7 @@ if TYPE_CHECKING:
 
     from app.config import Settings
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 
 def _check_single_worker_mode() -> None: