Add sync engine, web UI, Docker setup, and tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/helpers.py

@@ -0,0 +1,37 @@
+"""
+Shared test helpers for the Outline Sync Web UI test suite.
+Import directly: from tests.helpers import make_mock_process
+"""
+
+from unittest.mock import AsyncMock, MagicMock
+
+
+class _AsyncLineIter:
+    """Proper async iterator for mocking asyncio subprocess stdout."""
+
+    def __init__(self, lines: list[str]):
+        self._iter = iter(lines)
+
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self) -> bytes:
+        try:
+            return (next(self._iter) + "\n").encode()
+        except StopIteration:
+            raise StopAsyncIteration
+
+
+def make_mock_process(stdout_lines: list[str], returncode: int = 0) -> MagicMock:
+    """
+    Build a mock asyncio subprocess whose stdout is a proper async iterable.
+
+    Usage in tests:
+        with patch("webui.spawn_sync_subprocess") as mock_spawn:
+            mock_spawn.return_value = make_mock_process(["Done."])
+    """
+    proc = MagicMock()
+    proc.returncode = returncode
+    proc.stdout = _AsyncLineIter(stdout_lines)
+    proc.wait = AsyncMock(return_value=returncode)
+    return proc