fix: lowercase container name filter for case-sensitive docker ps

- get_container_id() and get_container_name() now lowercase the
  instance name before passing to docker ps --filter, because
  Docker container names are lowercase internally and the filter
  is case-sensitive. This caused container_id to never be captured
  when instance.name contained uppercase chars (e.g. 'Headquarter'),
  breaking terminal WebSocket connections.

- Also guard proc.stdout being None in start_cloudflared_tunnel().

- Add unit tests for get_container_id and get_container_name.

Quality gates: pytest (14 passed), python clean

apps/api/tests/unit/test_docker_service.py

@@ -0,0 +1,52 @@
+"""Unit tests for docker service utilities."""
+
+from unittest.mock import MagicMock, patch
+
+from src.services.docker import get_container_id, get_container_name
+
+
+class TestGetContainerId:
+    """Tests for get_container_id."""
+
+    @patch("subprocess.run")
+    def test_lowercases_name_for_filter(self, mock_run) -> None:
+        """Docker ps name filter is case-sensitive; we must lowercase."""
+        mock_run.return_value = MagicMock(returncode=0, stdout="abc123\n")
+
+        result = get_container_id("MyContainer-ABC")
+
+        assert result == "abc123"
+        call_args = mock_run.call_args[0][0]
+        # The filter must use lowercase
+        assert "name=mycontainer-abc" in call_args
+
+    @patch("subprocess.run")
+    def test_returns_none_when_not_found(self, mock_run) -> None:
+        mock_run.return_value = MagicMock(returncode=0, stdout="")
+
+        result = get_container_id("missing")
+
+        assert result is None
+
+
+class TestGetContainerName:
+    """Tests for get_container_name."""
+
+    @patch("subprocess.run")
+    def test_lowercases_name_for_filter(self, mock_run) -> None:
+        """Docker ps name filter is case-sensitive; we must lowercase."""
+        mock_run.return_value = MagicMock(returncode=0, stdout="mycontainer-abc\n")
+
+        result = get_container_name("MyContainer-ABC")
+
+        assert result == "mycontainer-abc"
+        call_args = mock_run.call_args[0][0]
+        assert "name=mycontainer-abc" in call_args
+
+    @patch("subprocess.run")
+    def test_returns_none_when_not_found(self, mock_run) -> None:
+        mock_run.return_value = MagicMock(returncode=0, stdout="")
+
+        result = get_container_name("missing")
+
+        assert result is None