test(routers): PR6 T6.7 — POST /api/threads workspace_id integration

`create_thread` in `app/gateway/routers/threads.py` already relies on
the AUTO sentinel default for ``ThreadMetaStore.create``, so once T6.1
landed the workspace_id was implicitly carried from the contextvar.
This commit adds the integration coverage: TestClient + stub auth
middleware + ``MemoryThreadMetaStore`` round-trip showing the persisted
row carries the right ``workspace_id`` under two distinct workspaces.

No production code change — the route wiring was correct, but the
guarantee was previously only proven at the unit level.

backend/tests/test_threads_post_workspace.py

@@ -0,0 +1,59 @@
+"""PR6 T6.7 — POST /api/threads writes workspace_id from contextvar.
+
+The `routers/threads.py:create_thread` path delegates to
+``ThreadMetaStore.create`` *without* an explicit ``workspace_id`` — it
+relies on the AUTO sentinel pulling the value from the active workspace
+contextvar that AuthMiddleware (or the test stub) sets. This test
+covers the integration through the FastAPI TestClient stack: post a
+thread under workspace A, then verify the persisted record carries
+``workspace_id="ws-alpha"``.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+
+from _router_auth_helpers import make_authed_test_app
+from fastapi.testclient import TestClient
+from langgraph.checkpoint.memory import InMemorySaver
+from langgraph.store.memory import InMemoryStore
+
+from app.gateway.auth.models import ActiveWorkspace
+from app.gateway.routers import threads
+from deerflow.persistence.thread_meta.memory import MemoryThreadMetaStore
+
+
+def _workspace_factory(wid: str) -> Callable[[], ActiveWorkspace]:
+    def _factory() -> ActiveWorkspace:
+        return ActiveWorkspace(id=wid, role="owner")
+
+    return _factory
+
+
+def _build_app(workspace_id: str):
+    app = make_authed_test_app(workspace_factory=_workspace_factory(workspace_id))
+    store = InMemoryStore()
+    checkpointer = InMemorySaver()
+    app.state.store = store
+    app.state.checkpointer = checkpointer
+    app.state.thread_store = MemoryThreadMetaStore(store)
+    app.include_router(threads.router)
+    return app, store
+
+
+def test_post_thread_stamps_workspace_id_from_contextvar():
+    app, store = _build_app("ws-alpha")
+    with TestClient(app) as client:
+        response = client.post("/api/threads", json={"thread_id": "t1", "metadata": {}})
+    assert response.status_code == 200, response.text
+
+    item = store.get(("threads",), "t1")
+    assert item is not None
+    assert item.value["workspace_id"] == "ws-alpha"
+
+
+def test_post_thread_under_different_workspace():
+    app, store = _build_app("ws-beta")
+    with TestClient(app) as client:
+        client.post("/api/threads", json={"thread_id": "t2", "metadata": {}})
+    assert store.get(("threads",), "t2").value["workspace_id"] == "ws-beta"