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/tests/logging_capture.py

@@ -0,0 +1,70 @@
+"""Test utilities for capturing stdlib log records."""
+
+from __future__ import annotations
+
+import logging
+from collections.abc import Generator
+from contextlib import contextmanager
+from typing import Any
+
+
+class _CaptureHandler(logging.Handler):
+    """Handler that stores every emitted record as a dict."""
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.records: list[dict[str, Any]] = []
+
+    def emit(self, record: logging.LogRecord) -> None:
+        entry: dict[str, Any] = {
+            "event": record.getMessage(),
+            "level": record.levelname.lower(),
+            "logger": record.name,
+        }
+        # Merge extra fields attached to the record.
+        std_attrs = {
+            "name",
+            "msg",
+            "args",
+            "levelname",
+            "levelno",
+            "pathname",
+            "filename",
+            "module",
+            "exc_info",
+            "exc_text",
+            "stack_info",
+            "lineno",
+            "funcName",
+            "created",
+            "msecs",
+            "relativeCreated",
+            "thread",
+            "threadName",
+            "processName",
+            "process",
+            "message",
+            "asctime",
+            "taskName",
+        }
+        for key, value in record.__dict__.items():
+            if key not in std_attrs:
+                entry[key] = value
+        self.records.append(entry)
+
+
+@contextmanager
+def capture_logs() -> Generator[list[dict[str, Any]], None, None]:
+    """Capture all log records emitted inside the context.
+
+    Yields a list of dicts, each representing a log entry with keys
+    ``event``, ``level``, ``logger`` and any extra fields.
+    """
+    handler = _CaptureHandler()
+    handler.setLevel(logging.DEBUG)
+    root = logging.getLogger()
+    root.addHandler(handler)
+    try:
+        yield handler.records
+    finally:
+        root.removeHandler(handler)