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/services/geo_cache.py

@@ -21,7 +21,7 @@ import time
 from typing import TYPE_CHECKING
 
 import aiohttp
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.models.geo import GeoInfo
 from app.repositories import geo_cache_repo
@@ -33,7 +33,7 @@ if TYPE_CHECKING:
     import geoip2.database
     import geoip2.errors
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # ---------------------------------------------------------------------------
 # Constants
@@ -208,9 +208,9 @@ class GeoCache:
         Returns:
             A dict with ``resolved`` and ``total`` counts.
         """
-        import structlog  # noqa: PLC0415
+        from app.utils.logging_compat import get_logger
 
-        log = structlog.get_logger()
+        log = get_logger(__name__)
         unresolved = await self.get_unresolved_ips(db)
         if not unresolved:
             return {"resolved": 0, "total": 0}