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/routers/config_misc.py

@@ -4,7 +4,7 @@ import shlex
 from pathlib import Path
 from typing import Annotated
 
-import structlog
+from app.utils.logging_compat import get_logger
 from fastapi import APIRouter, Depends, Query, Request, status
 
 from app.config import get_settings
@@ -37,7 +37,7 @@ from app.services import (
 )
 from app.utils.constants import CSRF_HEADER_NAME, CSRF_HEADER_VALUE, RATE_LIMIT_CONFIG_UPDATE_REQUESTS
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 router: APIRouter = APIRouter(tags=["Config Misc"])
 
@@ -60,11 +60,11 @@ def _check_config_update_rate_limit(
         _CONFIG_UPDATE_BUCKET, client_ip, RATE_LIMIT_CONFIG_UPDATE_REQUESTS, _MINUTE
     )
     if not is_allowed:
-        import structlog
+        from app.utils.logging_compat import get_logger
 
         from app.exceptions import RateLimitError
 
-        log = structlog.get_logger()
+        log = get_logger(__name__)
         log.warning(
             "config_update_rate_limit_exceeded",
             client_ip=client_ip,