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/json_formatter.py

@@ -0,0 +1,85 @@
+"""JSON formatter for stdlib logging that preserves extra fields.
+
+A single logging.Formatter subclass that serialises any keyword arguments
+passed via ``extra=`` into the JSON output alongside the standard record
+attributes.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from datetime import datetime, timezone
+from typing import Any
+
+# Attributes that belong to the standard LogRecord and should NOT be
+# treated as user-supplied extra fields.
+_STD_RECORD_ATTRS: frozenset[str] = frozenset(
+    {
+        "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",
+    }
+)
+
+
+class JSONFormatter(logging.Formatter):
+    """Format log records as JSON lines, including extra fields.
+
+    Usage::
+
+        handler = logging.StreamHandler()
+        handler.setFormatter(JSONFormatter())
+        logging.getLogger().addHandler(handler)
+
+    Output keys:
+        - ``event``      – the log message
+        - ``level``      – lower-cased level name
+        - ``timestamp``  – ISO-8601 UTC timestamp
+        - ``logger``     – logger name
+        - any ``extra`` fields supplied by the caller
+    """
+
+    def format(self, record: logging.LogRecord) -> str:
+        """Return a JSON string for *record*."""
+        log_dict: dict[str, Any] = {
+            "event": record.getMessage(),
+            "level": record.levelname.lower(),
+            "timestamp": (
+                datetime.fromtimestamp(record.created, tz=timezone.utc).isoformat()
+            ),
+            "logger": record.name,
+        }
+
+        # Merge any extra fields attached to the record.
+        for key, value in record.__dict__.items():
+            if key not in _STD_RECORD_ATTRS:
+                log_dict[key] = value
+
+        # Include exception info when present.
+        if record.exc_info and not record.exc_text:
+            record.exc_text = self.formatException(record.exc_info)
+        if record.exc_text:
+            log_dict["exception"] = record.exc_text
+
+        return json.dumps(log_dict, default=str)