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/bans.py

@@ -53,9 +53,9 @@ def _check_ban_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(
             "bans_ban_rate_limit_exceeded",
             client_ip=client_ip,
@@ -82,9 +82,9 @@ def _check_unban_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(
             "bans_unban_rate_limit_exceeded",
             client_ip=client_ip,