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/rate_limiter_cleanup.py

@@ -18,7 +18,7 @@ from __future__ import annotations
 import uuid
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.tasks.timeout_utils import run_with_timeout
 from app.utils.correlation import get_correlation_id, reset_correlation_id, set_correlation_id
@@ -26,7 +26,7 @@ from app.utils.correlation import get_correlation_id, reset_correlation_id, set_
 if TYPE_CHECKING:
     from fastapi import FastAPI
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 #: How often the cleanup job fires (seconds). Chosen to balance memory
 #: management against CPU overhead. A 30-minute interval handles typical
@@ -67,16 +67,6 @@ async def _do_cleanup_with_app(app: FastAPI) -> None:
     """Inner cleanup logic that runs with correlation context set."""
 
     async def _do_cleanup() -> None:
-        login_limiter = getattr(app.state, "login_rate_limiter", None)
-        if login_limiter is None:
-            log.warning(
-                "rate_limiter_cleanup_skipped",
-                correlation_id=get_correlation_id(),
-                reason="login_rate_limiter not found on app.state",
-            )
-        else:
-            login_limiter.cleanup_expired()
-
         global_limiter = getattr(app.state, "global_rate_limiter", None)
         if global_limiter is None:
             log.warning(