收敛用户角色并共享项目库

- 后端限制系统只保留默认 admin 管理员，新建用户固定为标注员，并拒绝观察员或额外管理员角色。

- 将项目、帧、媒体解析、AI 标注、任务、Dashboard 和导出接口改为共享项目库访问，标注员具备同等项目管理和标注能力。

- 前端用户管理移除角色选择和观察员入口，只展示唯一管理员与标注员状态。

- 更新后端/前端测试，覆盖唯一 admin、旧 viewer 归一为标注员、用户删除和共享项目库访问。

- 同步更新 AGENTS 与 doc 文档中的角色权限、共享项目库和测试计划说明。

backend/routers/tasks.py

@@ -33,13 +33,11 @@ def _now() -> datetime:
 
 
 def _get_task_or_404(task_id: int, db: Session, current_user: User) -> ProcessingTask:
+    _ = current_user
     task = (
         db.query(ProcessingTask)
         .outerjoin(Project, Project.id == ProcessingTask.project_id)
-        .filter(
-            ProcessingTask.id == task_id,
-            (ProcessingTask.project_id.is_(None)) | (Project.owner_user_id == current_user.id),
-        )
+        .filter(ProcessingTask.id == task_id)
         .first()
     )
     if not task:
@@ -60,9 +58,8 @@ def list_tasks(
     current_user: User = Depends(get_current_user),
 ) -> List[ProcessingTask]:
     """Return recent background processing tasks."""
-    query = db.query(ProcessingTask).outerjoin(Project, Project.id == ProcessingTask.project_id).filter(
-        (ProcessingTask.project_id.is_(None)) | (Project.owner_user_id == current_user.id)
-    )
+    _ = current_user
+    query = db.query(ProcessingTask).outerjoin(Project, Project.id == ProcessingTask.project_id)
     if project_id is not None:
         query = query.filter(ProcessingTask.project_id == project_id)
     if status is not None:
@@ -130,10 +127,7 @@ def retry_task(
     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,
-        Project.owner_user_id == current_user.id,
-    ).first()
+    project = db.query(Project).filter(Project.id == previous.project_id).first()
     if not project:
         raise HTTPException(status_code=404, detail="Project not found")
     is_propagation_task = previous.task_type == "propagate_masks"