feat: Langflow authoring pipeline with tests

- LangflowClient (list/export, x-api-key auth, error mapping)
- import_from_langflow: export -> versioned definition -> LFX deploy
- POST /api/flows/import, GET /api/flows/langflow/available
- scripts/deploy_flow.py CLI
- 25 tests green (Langflow faked via dependency override + respx
  client tests; respx passthrough proven unreliable for LFX traffic)

backend/tests/conftest.py

@@ -166,6 +166,35 @@ def lfx_server(tmp_path_factory):
         proc.kill()
 
 
+@pytest.fixture()
+def override_langflow():
+    """Replace the Langflow dependency with a fake (real LFX stays untouched).
+
+    Usage: override_langflow(export_doc) -- the fake export_flow() returns the
+    doc, or raises LangflowNotFound/Unavailable for status=404/0.
+    """
+    from app.flows.api import get_langflow
+    from app.flows.langflow_client import LangflowNotFound, LangflowUnavailable
+    from app.main import app
+
+    def _override(export_doc=None, status=200):
+        class FakeLangflow:
+            async def export_flow(self, flow_id):
+                if status == 404:
+                    raise LangflowNotFound(f"flow {flow_id} not found")
+                if status == 0:
+                    raise LangflowUnavailable("Langflow unreachable")
+                return export_doc
+
+            async def list_flows(self):
+                return [{"id": "fake-1", "name": "fake"}]
+
+        app.dependency_overrides[get_langflow] = lambda: FakeLangflow()
+
+    yield _override
+    app.dependency_overrides.pop(get_langflow, None)
+
+
 @pytest.fixture(scope="session")
 def live_server():
     """Real uvicorn server (ephemeral port) so LFX flows can call back