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/dashboard.py

@@ -58,12 +58,12 @@ def get_dashboard_overview(
     current_user: User = Depends(get_current_user),
 ) -> dict[str, Any]:
     """Return live dashboard data derived from persisted backend records."""
-    owned_project_ids_query = db.query(Project.id).filter(Project.owner_user_id == current_user.id)
-    project_count = db.query(func.count(Project.id)).filter(Project.owner_user_id == current_user.id).scalar() or 0
-    frame_count = db.query(func.count(Frame.id)).filter(Frame.project_id.in_(owned_project_ids_query)).scalar() or 0
+    shared_project_ids_query = db.query(Project.id)
+    project_count = db.query(func.count(Project.id)).scalar() or 0
+    frame_count = db.query(func.count(Frame.id)).filter(Frame.project_id.in_(shared_project_ids_query)).scalar() or 0
     annotation_count = (
         db.query(func.count(Annotation.id))
-        .filter(Annotation.project_id.in_(owned_project_ids_query))
+        .filter(Annotation.project_id.in_(shared_project_ids_query))
         .scalar()
         or 0
     )
@@ -76,7 +76,6 @@ def get_dashboard_overview(
     active_task_count = (
         db.query(func.count(ProcessingTask.id))
         .outerjoin(Project, Project.id == ProcessingTask.project_id)
-        .filter((ProcessingTask.project_id.is_(None)) | (Project.owner_user_id == current_user.id))
         .filter(ProcessingTask.status.in_(ACTIVE_TASK_STATUSES))
         .scalar()
         or 0
@@ -84,14 +83,12 @@ def get_dashboard_overview(
 
     projects = (
         db.query(Project)
-        .filter(Project.owner_user_id == current_user.id)
         .order_by(Project.updated_at.desc())
         .all()
     )
     recent_tasks = (
         db.query(ProcessingTask)
         .outerjoin(Project, Project.id == ProcessingTask.project_id)
-        .filter((ProcessingTask.project_id.is_(None)) | (Project.owner_user_id == current_user.id))
         .order_by(ProcessingTask.created_at.desc())
         .limit(50)
         .all()
@@ -120,7 +117,7 @@ def get_dashboard_overview(
 
     recent_annotations = (
         db.query(Annotation)
-        .filter(Annotation.project_id.in_(owned_project_ids_query))
+        .filter(Annotation.project_id.in_(shared_project_ids_query))
         .order_by(Annotation.updated_at.desc())
         .limit(10)
         .all()