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/regex_validator.py

@@ -11,7 +11,7 @@ import signal
 from contextlib import contextmanager
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 try:
     from regexploit.ast.sre import SreOpParser
@@ -25,7 +25,7 @@ except ImportError:
 if TYPE_CHECKING:
     from collections.abc import Generator
 
-logger = structlog.get_logger()
+logger = get_logger(__name__)
 
 # Constants for regex validation
 MAX_REGEX_LENGTH = 1000