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/scripts/deploy_flow.py

@@ -0,0 +1,54 @@
+"""Import a flow from Langflow into our backend via the running API.
+
+Usage:
+    uv run python scripts/deploy_flow.py --api http://localhost:8000 \\
+        --langflow-id <uuid> [--name my-flow] [--author jane] [--no-deploy]
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+import urllib.request
+
+
+def post(api: str, path: str, body: dict) -> dict:
+    request = urllib.request.Request(
+        api.rstrip("/") + path,
+        data=json.dumps(body).encode(),
+        headers={"Content-Type": "application/json"},
+        method="POST",
+    )
+    try:
+        with urllib.request.urlopen(request) as response:
+            return {"status": response.status, "body": json.loads(response.read())}
+    except urllib.error.HTTPError as exc:
+        return {"status": exc.code, "body": exc.read().decode()[:2000]}
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--api", default="http://localhost:8000")
+    parser.add_argument("--langflow-id", required=True)
+    parser.add_argument("--name", default=None)
+    parser.add_argument("--author", default="")
+    parser.add_argument("--no-deploy", action="store_true")
+    args = parser.parse_args()
+
+    result = post(
+        args.api,
+        "/api/flows/import",
+        {
+            "langflow_flow_id": args.langflow_id,
+            "name": args.name,
+            "author": args.author,
+            "deploy": not args.no_deploy,
+        },
+    )
+    print(json.dumps(result, indent=2, ensure_ascii=False))
+    return 0 if result["status"] in (200, 201) else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())