feat: 建立 SAM2 标注闭环基线

- 打通工作区真实标注闭环：支持手工多边形、矩形、圆形、点区域和线段生成 mask，并可保存、回显、更新和删除后端 annotation。

- 增强 polygon 编辑器：支持顶点拖动、顶点删除、边中点插入、多 polygon 子区域选择编辑，以及区域合并和区域去除。

- 接入 GT mask 导入：后端支持二值/多类别 mask 拆分、contour 转 polygon、distance transform seed point，前端支持导入、回显和 seed point 拖动编辑。

- 完善导出能力：COCO JSON 导出对齐前端，PNG mask ZIP 同时包含单标注 mask、按 zIndex 融合的 semantic_frame 和 semantic_classes.json。

- 打通异步任务管理：新增任务取消、重试、失败详情接口与 Dashboard 控件，worker 支持取消状态检查并通过 Redis/WebSocket 推送 cancelled 事件。

- 对接 Dashboard 后端数据：概览统计、解析队列和实时流转记录从 FastAPI 聚合接口与 WebSocket 更新。

- 增强 AI 推理参数：前端发送 crop_to_prompt、auto_filter_background 和 min_score，后端支持点/框 prompt 局部裁剪推理、结果回映射和负向点/低分过滤。

- 接入 SAM3 基础设施：新增独立 Python 3.12 sam3 环境安装脚本、外部 worker helper、后端桥接和真实 Python/CUDA/包/HF checkpoint access 状态检测。

- 保留 SAM3 授权边界：当前官方 facebook/sam3 gated 权重未授权时状态接口会返回不可用，不伪装成可推理。

- 增强前端状态管理：新增 mask undo/redo 历史栈、AI 模型选择状态、保存状态 dirty/draft/saved 流转和项目状态归一化。

- 更新前端 API 封装：补充 annotation CRUD、GT mask import、mask ZIP export、task cancel/retry/detail、AI runtime status 和 prediction options。

- 更新 UI 控件：ToolsPalette、AISegmentation、VideoWorkspace 和 CanvasArea 接入真实操作、导入导出、撤销重做、任务控制和模型状态。

- 新增 polygon-clipping 依赖，用于前端区域 union/difference 几何运算。

- 完善后端 schemas/status/progress：补充 AI 模型外部状态字段、任务 cancelled 状态和进度事件 payload。

- 补充测试覆盖：新增后端任务控制、SAM3 桥接、GT mask、导出融合、AI options 测试；补充前端 Canvas、Dashboard、VideoWorkspace、ToolsPalette、API 和 store 测试。

- 更新 README、AGENTS 和 doc 文档：冻结当前需求/设计/测试计划，标注真实功能、剩余 Mock、SAM3 授权边界和后续实施顺序。

backend/routers/tasks.py

@@ -1,15 +1,45 @@
 """Processing task query endpoints."""
 
+import logging
+from datetime import datetime, timezone
 from typing import List
 
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.orm import Session
 
+from celery_app import celery_app
 from database import get_db
-from models import ProcessingTask
+from models import ProcessingTask, Project
+from progress_events import publish_task_progress_event
 from schemas import ProcessingTaskOut
+from statuses import (
+    PROJECT_STATUS_PARSING,
+    PROJECT_STATUS_PENDING,
+    PROJECT_STATUS_READY,
+    TASK_ACTIVE_STATUSES,
+    TASK_STATUS_CANCELLED,
+    TASK_STATUS_FAILED,
+    TASK_STATUS_QUEUED,
+)
+from worker_tasks import parse_project_media
 
 router = APIRouter(prefix="/api/tasks", tags=["Tasks"])
+logger = logging.getLogger(__name__)
+
+
+def _now() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+def _get_task_or_404(task_id: int, db: Session) -> ProcessingTask:
+    task = db.query(ProcessingTask).filter(ProcessingTask.id == task_id).first()
+    if not task:
+        raise HTTPException(status_code=404, detail="Task not found")
+    return task
+
+
+def _project_status_after_stop(project: Project) -> str:
+    return PROJECT_STATUS_READY if project.frames else PROJECT_STATUS_PENDING
 
 
 @router.get("", response_model=List[ProcessingTaskOut], summary="List processing tasks")
@@ -31,7 +61,78 @@ def list_tasks(
 @router.get("/{task_id}", response_model=ProcessingTaskOut, summary="Get processing task")
 def get_task(task_id: int, db: Session = Depends(get_db)) -> ProcessingTask:
     """Return one background task by id."""
-    task = db.query(ProcessingTask).filter(ProcessingTask.id == task_id).first()
-    if not task:
-        raise HTTPException(status_code=404, detail="Task not found")
+    return _get_task_or_404(task_id, db)
+
+
+@router.post("/{task_id}/cancel", response_model=ProcessingTaskOut, summary="Cancel processing task")
+def cancel_task(task_id: int, db: Session = Depends(get_db)) -> ProcessingTask:
+    """Cancel a queued/running background task and revoke the Celery job when possible."""
+    task = _get_task_or_404(task_id, db)
+    if task.status not in TASK_ACTIVE_STATUSES:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"Task is not cancellable in status: {task.status}",
+        )
+
+    if task.celery_task_id:
+        try:
+            celery_app.control.revoke(task.celery_task_id, terminate=True, signal="SIGTERM")
+        except Exception as exc:  # noqa: BLE001
+            logger.warning("Failed to revoke celery task %s: %s", task.celery_task_id, exc)
+
+    task.status = TASK_STATUS_CANCELLED
+    task.progress = 100
+    task.message = "任务已取消"
+    task.error = "Cancelled by user"
+    task.finished_at = _now()
+    if task.project:
+        task.project.status = _project_status_after_stop(task.project)
+
+    db.commit()
+    db.refresh(task)
+    publish_task_progress_event(task)
+    return task
+
+
+@router.post("/{task_id}/retry", response_model=ProcessingTaskOut, status_code=status.HTTP_202_ACCEPTED, summary="Retry processing task")
+def retry_task(task_id: int, db: Session = Depends(get_db)) -> ProcessingTask:
+    """Create a fresh queued task from a failed or cancelled task."""
+    previous = _get_task_or_404(task_id, db)
+    if previous.status not in {TASK_STATUS_FAILED, TASK_STATUS_CANCELLED}:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"Task is not retryable in status: {previous.status}",
+        )
+    if previous.project_id is None:
+        raise HTTPException(status_code=400, detail="Task has no project_id")
+
+    project = db.query(Project).filter(Project.id == previous.project_id).first()
+    if not project:
+        raise HTTPException(status_code=404, detail="Project not found")
+    if not project.video_path:
+        raise HTTPException(status_code=400, detail="Project has no media uploaded")
+
+    payload = dict(previous.payload or {})
+    payload.setdefault("source_type", project.source_type or "video")
+    payload["retry_of"] = previous.id
+
+    task = ProcessingTask(
+        task_type=previous.task_type,
+        status=TASK_STATUS_QUEUED,
+        progress=0,
+        message=f"重试任务已入队（源任务 #{previous.id}）",
+        project_id=project.id,
+        payload=payload,
+    )
+    project.status = PROJECT_STATUS_PARSING
+    db.add(task)
+    db.commit()
+    db.refresh(task)
+    publish_task_progress_event(task)
+
+    async_result = parse_project_media.delay(task.id)
+    task.celery_task_id = async_result.id
+    db.commit()
+    db.refresh(task)
+    publish_task_progress_event(task)
     return task