调整项目库拆帧与长帧序列加载

- 删除项目库右上角独立新建项目入口，保留导入视频/DICOM 自动建项目流程

- 视频项目支持已生成帧后的重新生成帧入口，并提示会清空旧帧、标注和 mask

- 后端重新拆帧任务开始前清理旧帧、旧标注和旧 mask，避免重复帧序列

- 项目帧列表接口默认返回完整帧序列，避免工作区总帧数被 1000 条默认 limit 截断

- 增加可选 docker-compose.gpu.yml，并补充 Docker 使用本机 GPU 的前提和启动说明

- 更新项目库、API 映射、恢复演示文案、后端媒体/项目测试和前端文档

backend/tests/test_projects.py

@@ -50,6 +50,29 @@ def test_project_crud_and_frames(client, monkeypatch):
     assert client.get(f"/api/projects/{project_id}").status_code == 404
 
 
+def test_list_project_frames_returns_more_than_default_timeline_window(client, db_session, monkeypatch):
+    monkeypatch.setattr("routers.projects.get_presigned_url", lambda key, expires=3600: f"http://storage/{key}")
+    created = client.post("/api/projects", json={"name": "Long Sequence", "status": "ready"})
+    assert created.status_code == 201
+    project_id = created.json()["id"]
+
+    db_session.add_all([
+        Frame(project_id=project_id, frame_index=index, image_url=f"frames/{index}.jpg")
+        for index in range(1001)
+    ])
+    db_session.commit()
+
+    frames = client.get(f"/api/projects/{project_id}/frames")
+    assert frames.status_code == 200
+    body = frames.json()
+    assert len(body) == 1001
+    assert body[-1]["frame_index"] == 1000
+
+    limited = client.get(f"/api/projects/{project_id}/frames?limit=2")
+    assert limited.status_code == 200
+    assert [frame["frame_index"] for frame in limited.json()] == [0, 1]
+
+
 def test_delete_project_cascades_related_records(client, db_session):
     created = client.post("/api/projects", json={"name": "Cascade Delete"})
     assert created.status_code == 201