feat: 完善分割工作区传播与交互闭环

功能增加：新增后端传播任务执行器，支持异步自动传播、传播进度、结果统计、取消/重试状态同步。

功能增加：传播请求支持指定 SAM2.1 tiny/small/base+/large 权重，并记录 seed mask、source annotation 和传播范围。

功能增加：传播逻辑增加 seed 签名，未变化的 mask 二次传播会跳过，已变化的 mask 会先清理旧自动传播结果再重新生成，避免重复重叠。

功能增加：工作区增加传播范围二次选择、传播进度提示、人工/AI 标注帧红色标识、自动传播帧蓝色标识和当前帧双层边框。

功能增加：新增临时提示组件，让工具操作提示自动消失且不阻塞后续操作。

功能增加：补充项目删除、模板删除、任务失败详情、任务取消/重试等前后端联动状态。

功能增加：新增安装部署文档，补充当前需求冻结、设计冻结、接口契约、测试计划和 AGENTS/README 项目说明。

Bugfix：修复自动传播接口 404、传播后看不到任务进度、传播结果重复堆叠和已编辑帧提示不清晰的问题。

Bugfix：修复 AI 分割框选/点选交互、单候选 mask、删除选点、工作区保存与候选 mask 推送相关问题。

Bugfix：修复 Canvas 多边形顶点拖动告警、工具栏提示缺失、项目库 FPS 展示和若干 UI 文案/可用性问题。

测试：补充 AI 分割、Canvas、Dashboard、FrameTimeline、ProjectLibrary、TemplateRegistry、ToolsPalette、VideoWorkspace、API 和后端任务/AI/dashboard 测试。

验证：npm run lint；npm run test:run；python -m pytest backend/tests -q。

backend/routers/ai.py

@@ -12,7 +12,7 @@ from sqlalchemy.orm import Session
 
 from database import get_db
 from minio_client import download_file
-from models import Project, Frame, Template, Annotation
+from models import Project, Frame, Template, Annotation, ProcessingTask
 from schemas import (
     AiRuntimeStatus,
     MaskAnalysisRequest,
@@ -21,10 +21,15 @@ from schemas import (
     PredictResponse,
     PropagateRequest,
     PropagateResponse,
+    PropagateTaskRequest,
+    ProcessingTaskOut,
     AnnotationOut,
     AnnotationCreate,
     AnnotationUpdate,
 )
+from progress_events import publish_task_progress_event
+from statuses import TASK_STATUS_QUEUED
+from worker_tasks import propagate_project_masks
 from services.sam_registry import ModelUnavailableError, sam_registry
 
 logger = logging.getLogger(__name__)
@@ -586,6 +591,66 @@ def propagate(payload: PropagateRequest, db: Session = Depends(get_db)) -> dict:
     }
 
 
+@router.post(
+    "/propagate/task",
+    status_code=status.HTTP_202_ACCEPTED,
+    response_model=ProcessingTaskOut,
+    summary="Queue a background video propagation task",
+)
+def queue_propagate_task(payload: PropagateTaskRequest, db: Session = Depends(get_db)) -> ProcessingTaskOut:
+    """Queue multiple seed/direction propagation steps as one background task."""
+    project = db.query(Project).filter(Project.id == payload.project_id).first()
+    if not project:
+        raise HTTPException(status_code=404, detail="Project not found")
+
+    source_frame = db.query(Frame).filter(
+        Frame.id == payload.frame_id,
+        Frame.project_id == payload.project_id,
+    ).first()
+    if not source_frame:
+        raise HTTPException(status_code=404, detail="Frame not found")
+
+    if not payload.steps:
+        raise HTTPException(status_code=400, detail="Propagation task requires at least one step")
+
+    try:
+        model_id = sam_registry.normalize_model_id(payload.model)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+
+    for step in payload.steps:
+        direction = step.direction.lower()
+        if direction not in {"forward", "backward"}:
+            raise HTTPException(status_code=400, detail="direction must be forward or backward")
+        seed = step.seed.model_dump(exclude_none=True)
+        if not (seed.get("polygons") or seed.get("bbox") or seed.get("points")):
+            raise HTTPException(status_code=400, detail="Propagation requires seed polygons, bbox, or points")
+
+    task_payload = payload.model_dump(exclude_none=True)
+    task_payload["model"] = model_id
+    task = ProcessingTask(
+        task_type="propagate_masks",
+        status=TASK_STATUS_QUEUED,
+        progress=0,
+        message="自动传播任务已入队",
+        project_id=payload.project_id,
+        payload=task_payload,
+    )
+    db.add(task)
+    db.commit()
+    db.refresh(task)
+    publish_task_progress_event(task)
+
+    async_result = propagate_project_masks.delay(task.id)
+    task.celery_task_id = async_result.id
+    db.commit()
+    db.refresh(task)
+    publish_task_progress_event(task)
+
+    logger.info("Queued propagation task id=%s project_id=%s celery_id=%s", task.id, payload.project_id, async_result.id)
+    return task
+
+
 @router.post(
     "/auto",
     response_model=PredictResponse,