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/tasks/session_cleanup.py

@@ -16,7 +16,7 @@ from __future__ import annotations
 import uuid
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.repositories import session_repo
 from app.tasks.db import task_db
@@ -30,7 +30,7 @@ if TYPE_CHECKING:
 
     from app.config import Settings
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 #: How often the cleanup job fires (seconds).  Configurable tuning constant.
 SESSION_CLEANUP_INTERVAL: int = 6 * 60 * 60  # 6 hours