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/action_config.py

@@ -41,9 +41,9 @@ def _check_action_update_rate_limit(
     )
     if not is_allowed:
         from app.exceptions import RateLimitError
-        import structlog
+        from app.utils.logging_compat import get_logger
 
-        log = structlog.get_logger()
+        log = get_logger(__name__)
         log.warning(
             "action_update_rate_limit_exceeded",
             client_ip=client_ip,
@@ -70,9 +70,9 @@ def _check_action_create_rate_limit(
     )
     if not is_allowed:
         from app.exceptions import RateLimitError
-        import structlog
+        from app.utils.logging_compat import get_logger
 
-        log = structlog.get_logger()
+        log = get_logger(__name__)
         log.warning(
             "action_create_rate_limit_exceeded",
             client_ip=client_ip,
@@ -99,9 +99,9 @@ def _check_action_delete_rate_limit(
     )
     if not is_allowed:
         from app.exceptions import RateLimitError
-        import structlog
+        from app.utils.logging_compat import get_logger
 
-        log = structlog.get_logger()
+        log = get_logger(__name__)
         log.warning(
             "action_delete_rate_limit_exceeded",
             client_ip=client_ip,