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/utils/logging_compat.py

@@ -0,0 +1,63 @@
+"""Compatibility shim providing keyword-argument logging API on top of stdlib logging.
+
+This module lets the rest of the codebase keep the keyword-argument logging
+style (``log.info("event", key=value)``) while using only the Python standard
+library ``logging`` module underneath.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+
+class _CompatLogger:
+    """Wraps a stdlib :class:`logging.Logger` to accept keyword arguments."""
+
+    def __init__(self, logger: logging.Logger) -> None:
+        self._logger = logger
+
+    def _log(self, level: int, event: str, **kwargs: Any) -> None:
+        exc_info = kwargs.pop("exc_info", None)
+        extra = kwargs if kwargs else None
+        self._logger.log(level, event, exc_info=exc_info, extra=extra)
+
+    def debug(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.DEBUG, event, **kwargs)
+
+    def info(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.INFO, event, **kwargs)
+
+    def warning(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.WARNING, event, **kwargs)
+
+    def warn(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.WARNING, event, **kwargs)
+
+    def error(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.ERROR, event, **kwargs)
+
+    def critical(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.CRITICAL, event, **kwargs)
+
+    def exception(self, event: str, **kwargs: Any) -> None:
+        self._log(logging.ERROR, event, exc_info=True, **kwargs)
+
+    def bind(self, **kwargs: Any) -> "_CompatLogger":
+        """Return a new logger with bound context (no-op for stdlib)."""
+        return self
+
+
+def get_logger(name: str | None = None) -> _CompatLogger:
+    """Get a compatibility logger wrapping the stdlib logger for *name*.
+
+    If *name* is ``None`` the caller's module name is used.
+    """
+    if name is None:
+        import sys
+
+        # Walk up the stack to find the caller's module.
+        frame = sys._getframe(1)
+        module = frame.f_globals.get("__name__", "__main__")
+        name = module
+    return _CompatLogger(logging.getLogger(name))