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/routers/projects.py

@@ -1,7 +1,7 @@
 """Project and Frame CRUD endpoints."""
 
 import logging
-from typing import List
+from typing import List, Optional
 
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.orm import Session
@@ -261,7 +261,7 @@ def create_frame(
 def list_frames(
     project_id: int,
     skip: int = 0,
-    limit: int = 1000,
+    limit: Optional[int] = None,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ) -> List[Frame]:
@@ -270,14 +270,15 @@ def list_frames(
     if not project:
         raise HTTPException(status_code=404, detail="Project not found")
 
-    frames = (
+    query = (
         db.query(Frame)
         .filter(Frame.project_id == project_id)
         .order_by(Frame.frame_index)
         .offset(skip)
-        .limit(limit)
-        .all()
     )
+    if limit is not None:
+        query = query.limit(limit)
+    frames = query.all()
     for frame in frames:
         frame.image_url = get_presigned_url(frame.image_url, expires=3600)
     return frames