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/middleware/csrf.py

@@ -25,7 +25,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 from fastapi import status
 from fastapi.responses import JSONResponse
 from starlette.middleware.base import BaseHTTPMiddleware
@@ -38,7 +38,7 @@ if TYPE_CHECKING:
     from starlette.requests import Request
     from starlette.responses import Response as StarletteResponse
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # HTTP methods that require CSRF protection.
 _CSRF_PROTECTED_METHODS: frozenset[str] = frozenset({"POST", "PUT", "DELETE", "PATCH"})