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/blocklist.py

@@ -22,7 +22,7 @@ registered *before* the ``/{id}`` routes so FastAPI resolves them correctly.
 
 from __future__ import annotations
 
-import structlog
+from app.utils.logging_compat import get_logger
 from fastapi import APIRouter, Depends, Query, Request, status
 
 from app.dependencies import (
@@ -64,7 +64,7 @@ _BLOCKLIST_IMPORT_BUCKET = "blocklist:import"
 # 3600 seconds per hour
 _HOUR = 3600
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 
 def _check_blocklist_import_rate_limit(