Finish external HTTP client resilience: add shared aiohttp config, retry support, and update task status

backend/tests/test_services/test_blocklist_service.py

@@ -125,6 +125,27 @@ class TestPreview:
         with pytest.raises(ValueError, match="HTTP 404"):
             await blocklist_service.preview_source("https://bad.test/", session)
 
+    async def test_preview_retries_transient_errors(self) -> None:
+        """preview_source retries transient network failures before succeeding."""
+        content = "1.2.3.4\n"
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.text = AsyncMock(return_value=content)
+        mock_resp.content = AsyncMock()
+        mock_resp.content.read = AsyncMock(return_value=content.encode())
+
+        mock_ctx = AsyncMock()
+        mock_ctx.__aenter__.return_value = mock_resp
+        mock_ctx.__aexit__.return_value = False
+
+        session = MagicMock()
+        session.get = MagicMock(side_effect=[Exception("connection reset"), mock_ctx])
+
+        result = await blocklist_service.preview_source("https://test.test/ips.txt", session)
+
+        assert result.valid_count == 1
+        assert session.get.call_count == 2
+
     async def test_preview_limits_entries(self) -> None:
         """preview_source caps entries to sample_lines."""
         ips = "\n".join(f"1.2.3.{i}" for i in range(50))