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/app/flows/langflow_client.py

@@ -0,0 +1,66 @@
+"""Async client for the Langflow authoring server (visual designer).
+
+Only used for the authoring pipeline (list/export flows). Execution always
+goes through LFX (see lfx_client.py).
+"""
+
+from __future__ import annotations
+
+import httpx
+
+
+class LangflowError(Exception):
+    pass
+
+
+class LangflowNotFound(LangflowError):
+    pass
+
+
+class LangflowUnavailable(LangflowError):
+    pass
+
+
+class LangflowClient:
+    """Thin async wrapper around the Langflow `/api/v1/flows` endpoints."""
+
+    def __init__(self, base_url: str, api_key: str = "", timeout: float = 60.0) -> None:
+        self.base_url = base_url.rstrip("/")
+        self.api_key = api_key
+        self.timeout = timeout
+
+    def _headers(self) -> dict[str, str]:
+        return {"x-api-key": self.api_key} if self.api_key else {}
+
+    async def list_flows(self) -> list[dict]:
+        """List flows (header mode keeps payloads small)."""
+        try:
+            async with httpx.AsyncClient(
+                base_url=self.base_url, timeout=self.timeout, headers=self._headers()
+            ) as client:
+                response = await client.get("/api/v1/flows/", params={"header_flows": "true"})
+                if response.status_code == 404:
+                    raise LangflowNotFound("flow list not found")
+                response.raise_for_status()
+                data = response.json()
+                return data if isinstance(data, list) else data.get("items", [])
+        except httpx.HTTPStatusError as exc:
+            raise LangflowError(f"Langflow error {exc.response.status_code}") from exc
+        except httpx.HTTPError as exc:
+            raise LangflowUnavailable(f"Langflow unreachable: {exc}") from exc
+
+    async def export_flow(self, langflow_flow_id: str) -> dict:
+        """Export one flow as Langflow JSON (FlowRead: id/name/description/data)."""
+        try:
+            async with httpx.AsyncClient(
+                base_url=self.base_url, timeout=self.timeout, headers=self._headers()
+            ) as client:
+                response = await client.get(f"/api/v1/flows/{langflow_flow_id}")
+                if response.status_code == 404:
+                    raise LangflowNotFound(f"flow {langflow_flow_id} not found")
+                response.raise_for_status()
+                return response.json()
+        except httpx.HTTPStatusError as exc:
+            raise LangflowError(f"Langflow error {exc.response.status_code}") from exc
+        except httpx.HTTPError as exc:
+            raise LangflowUnavailable(f"Langflow unreachable: {exc}") from exc