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/log_sanitizer.py

@@ -1,7 +1,7 @@
 """Log sanitization utilities for preventing sensitive data leakage.
 
 All external output (subprocess, API responses, config data) passed to
-structlog MUST be sanitized first. This module provides the canonical
+logging MUST be sanitized first. This module provides the canonical
 sanitize_for_logging() function used across the codebase.
 """