add explain module

sentence_api/tests/test_api.py

@@ -80,6 +80,70 @@ def test_invalid_hash_is_rejected(tmp_path):
     assert response.status_code == 422
 
 
+def test_sentence_explanation_requires_login(tmp_path):
+    index_path = tmp_path / "boundaries.json"
+    index_path.write_text("{}", encoding="utf-8")
+    settings = replace(
+        Settings.from_env(),
+        data_dir=tmp_path / "data",
+        legacy_boundaries_file=index_path,
+    )
+    settings.ensure_directories()
+    client = TestClient(
+        create_app(
+            BoundaryStore(index_path),
+            settings=settings,
+            repository=VideoRepository(settings.database_path),
+        )
+    )
+
+    response = client.post(
+        "/api/v1/sentences/explain",
+        json={"sentence_text": "Hello."},
+    )
+
+    assert response.status_code == 401
+
+
+def test_sentence_explanation_uses_server_side_ai_client(tmp_path, monkeypatch):
+    client = make_client(tmp_path)
+    requested = []
+
+    def explain(text, **kwargs):
+        requested.append((text, kwargs))
+        return "这句话表示打招呼。"
+
+    class StubAITutor:
+        pass
+
+    stub = StubAITutor()
+    stub.explain = explain
+    monkeypatch.setattr(client.app.state, "ai_tutor", stub)
+
+    response = client.post(
+        "/api/v1/sentences/explain",
+        json={
+            "sentence_text": "Hello.",
+            "course_title": "Greetings",
+            "sentence_index": 0,
+            "language": "en",
+        },
+    )
+
+    assert response.status_code == 200
+    assert response.json() == {"explanation": "这句话表示打招呼。"}
+    assert requested == [
+        (
+            "Hello.",
+            {
+                "course_title": "Greetings",
+                "sentence_index": 0,
+                "language": "en",
+            },
+        )
+    ]
+
+
 def test_oss_video_url_is_used_for_catalog_and_content(tmp_path):
     index_path = tmp_path / "boundaries.json"
     index_path.write_text("{}", encoding="utf-8")