feat: 完善 MVP-2 演示和进度体验

tests/unit/test_demo_seed.py

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+from sqlalchemy.orm import Session
+
+from app.demo_seed import seed_demo_data
+from app.models import Comment, ContentItem, Task
+from tests.helpers import make_test_client
+
+
+def test_seed_demo_data_is_idempotent_and_removes_sensitive_fields():
+    with make_test_client() as (_client, engine):
+        with Session(engine) as session:
+            task_id = seed_demo_data(session)
+            second_task_id = seed_demo_data(session)
+
+            task = session.get(Task, task_id)
+            comments = session.query(Comment).all()
+            items = session.query(ContentItem).all()
+
+    assert second_task_id == task_id
+    assert task is not None
+    assert task.id.startswith("demo-")
+    assert task.status == "success"
+    assert comments
+    assert all(comment.author is None for comment in comments)
+    assert all(comment.source_comment_id is None for comment in comments)
+    assert all(comment.raw_data == "{}" for comment in comments)
+    assert all(item.url is None for item in items)
+    assert all(item.raw_data == "{}" for item in items)