refactor(ban_service): extract _bans_by_country_load_data helper

Break up long function into focused helper. Load data logic separate from aggregation.

backend/app/main.py

@@ -22,6 +22,8 @@ if TYPE_CHECKING:
 
     from starlette.responses import Response as StarletteResponse
 
+    from app.models.response import ErrorMetadata
+
 import structlog
 from fastapi import FastAPI, HTTPException, Request, status
 from fastapi.exceptions import RequestValidationError
@@ -327,7 +329,7 @@ def _get_error_code(exc: Exception) -> str:
     return snake_case
 
 
-def _get_error_metadata(exc: Exception) -> dict[str, str | int | float | bool | None]:
+def _get_error_metadata(exc: Exception) -> ErrorMetadata:
     """Get structured metadata from an exception.
     
     Calls the exception's get_error_metadata() method if available.