Make retry handlers static methods

Convert handle_network_failure and handle_download_failure from instance methods to static methods. Hardcode retry params (max_retries, delays) instead of using instance state. Improves testability and removes implicit dependencies.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/unit/test_health.py

@@ -29,7 +29,7 @@ async def test_basic_health_check_no_startup_checks():
 
         assert isinstance(result, HealthStatus)
         assert result.status == "healthy"
-        assert result.version == "1.0.1"
+        assert result.version == "v1.3.6"
         assert result.service == "aniworld-api"
         assert result.timestamp is not None
         assert result.series_app_initialized is False