diff --git a/.env.example b/.env.example
index 231f2f5..fa54985 100644
--- a/.env.example
+++ b/.env.example
@@ -14,10 +14,10 @@ VITE_API_BASE_URL="http://192.168.3.11:8000"
 # Optional WebSocket override. If unset, it is derived from VITE_API_BASE_URL.
 VITE_WS_PROGRESS_URL="ws://192.168.3.11:8000/ws/progress"
 
-# Backend SAM runtime defaults. SAM 3 additionally requires the official sam3
-# package, Python 3.12+, PyTorch 2.7+, and a CUDA-capable GPU per Meta's repo.
-sam_default_model="sam2"
-sam_model_path="/home/wkmgc/Desktop/Seg_Server/models/sam2_hiera_tiny.pt"
-sam_model_config="configs/sam2/sam2_hiera_t.yaml"
+# Backend SAM runtime defaults. Current product exposes SAM 2.1 variants only.
+sam_default_model="sam2.1_hiera_tiny"
+sam_model_path="/home/wkmgc/Desktop/Seg_Server/models/sam2.1_hiera_tiny.pt"
+sam_model_config="configs/sam2.1/sam2.1_hiera_t.yaml"
 sam3_model_version="sam3"
 sam3_checkpoint_path="/home/wkmgc/Desktop/Seg_Server/sam3权重/sam3.pt"
+sam3_external_enabled=false
diff --git a/backend/config.py b/backend/config.py
index 6e48e9e..42da37d 100644
--- a/backend/config.py
+++ b/backend/config.py
@@ -24,7 +24,7 @@ class Settings(BaseSettings):
     sam_model_config: str = "configs/sam2.1/sam2.1_hiera_t.yaml"
     sam3_model_version: str = "sam3"
     sam3_checkpoint_path: str = "/home/wkmgc/Desktop/Seg_Server/sam3权重/sam3.pt"
-    sam3_external_enabled: bool = True
+    sam3_external_enabled: bool = False
     sam3_external_python: str = "/home/wkmgc/miniconda3/envs/sam3/bin/python"
     sam3_timeout_seconds: int = 300
     sam3_status_cache_seconds: int = 30
diff --git a/backend/main.py b/backend/main.py
index 1baf22b..5f7fd63 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -46,22 +46,17 @@ def _ensure_runtime_schema_columns() -> None:
         logger.warning("Runtime schema column check failed: %s", exc)
 
 
-def _seed_default_admin_and_ownership_sync() -> None:
-    """Ensure the default admin exists and owns legacy unassigned projects."""
-    from models import Project
+def _seed_default_admin_sync() -> None:
+    """Ensure the single default admin exists without rewriting project ownership metadata."""
     from routers.auth import ensure_default_admin
 
     db = SessionLocal()
     try:
         admin = ensure_default_admin(db)
-        db.query(Project).filter(Project.owner_user_id.is_(None)).update(
-            {"owner_user_id": admin.id},
-            synchronize_session=False,
-        )
         db.commit()
-        logger.info("Default admin ready; legacy projects assigned to user id=%s", admin.id)
+        logger.info("Default admin ready id=%s", admin.id)
     except Exception as exc:  # noqa: BLE001
-        logger.error("Failed to seed default admin or ownership: %s", exc)
+        logger.error("Failed to seed default admin: %s", exc)
     finally:
         db.close()
 
@@ -82,10 +77,7 @@ def _seed_default_project_sync() -> None:
     try:
         admin = ensure_default_admin(db)
         existing_video = db.query(Project).filter(Project.name == DEMO_VIDEO_PROJECT_NAME).first()
-        if existing_video is not None and existing_video.owner_user_id is None:
-            existing_video.owner_user_id = admin.id
-            db.commit()
-        elif existing_video is None and os.path.exists(settings.demo_video_path):
+        if existing_video is None and os.path.exists(settings.demo_video_path):
             video_project = create_unparsed_video_demo_project(
                 db,
                 owner=admin,
@@ -96,9 +88,6 @@ def _seed_default_project_sync() -> None:
 
         existing_dicom = db.query(Project).filter(Project.name == DEMO_DICOM_PROJECT_NAME).first()
         if existing_dicom is not None:
-            if existing_dicom.owner_user_id is None:
-                existing_dicom.owner_user_id = admin.id
-                db.commit()
             return
 
         if not demo_dicom_files(settings.demo_dicom_dir):
@@ -147,7 +136,7 @@ async def lifespan(app: FastAPI):
     try:
         Base.metadata.create_all(bind=engine)
         _ensure_runtime_schema_columns()
-        _seed_default_admin_and_ownership_sync()
+        _seed_default_admin_sync()
         logger.info("Database tables initialized.")
     except Exception as exc:  # noqa: BLE001
         logger.error("Database initialization failed: %s", exc)
diff --git a/backend/models.py b/backend/models.py
index e6e7271..268bbcb 100644
--- a/backend/models.py
+++ b/backend/models.py
@@ -25,7 +25,7 @@ class User(Base):
     id = Column(Integer, primary_key=True, index=True)
     username = Column(String(150), unique=True, index=True, nullable=False)
     password_hash = Column(String(255), nullable=False)
-    role = Column(String(50), default="admin", nullable=False)
+    role = Column(String(50), default="annotator", nullable=False)
     is_active = Column(Integer, default=1, nullable=False)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(
diff --git a/backend/tests/test_auth.py b/backend/tests/test_auth.py
index a7cda29..7c257d5 100644
--- a/backend/tests/test_auth.py
+++ b/backend/tests/test_auth.py
@@ -30,3 +30,37 @@ def test_business_routes_require_auth(app):
         response = unauthenticated.get("/api/projects")
 
     assert response.status_code == 401
+
+
+def test_default_admin_seed_does_not_claim_legacy_shared_projects(db_session):
+    from models import Project
+    from routers.auth import ensure_default_admin
+
+    project = Project(name="Legacy Shared Project", owner_user_id=None)
+    db_session.add(project)
+    db_session.commit()
+    db_session.refresh(project)
+
+    ensure_default_admin(db_session)
+    db_session.refresh(project)
+
+    assert project.owner_user_id is None
+
+
+def test_user_model_default_role_is_annotator(db_session):
+    from models import User
+    from routers.auth import hash_password
+
+    user = User(username="script-created", password_hash=hash_password("secret123"))
+    db_session.add(user)
+    db_session.commit()
+    db_session.refresh(user)
+
+    assert user.role == "annotator"
+
+
+def test_backend_runtime_defaults_match_current_product():
+    from config import settings
+
+    assert settings.sam_default_model == "sam2.1_hiera_tiny"
+    assert settings.sam3_external_enabled is False
diff --git a/doc/02-current-implementation-map.md b/doc/02-current-implementation-map.md
index 371beec..480b87a 100644
--- a/doc/02-current-implementation-map.md
+++ b/doc/02-current-implementation-map.md
@@ -72,7 +72,7 @@
 2. `UserAdmin.tsx` 调用 `GET/POST/PATCH/DELETE /api/admin/users` 完成标注员新增、停用/启用、改密码和删除用户；不提供观察员或第二个管理员入口。
 3. `UserAdmin.tsx` 调用 `GET /api/admin/audit-logs` 展示登录成功/失败以及用户管理操作审计。
 4. `UserAdmin.tsx` 危险区“恢复演示出厂设置”需要浏览器确认和输入 `RESET_DEMO_FACTORY`，随后调用 `POST /api/admin/demo-factory-reset`。
-5. 后端 `backend/routers/admin.py` 会阻止管理员删除、停用或降级自己，并阻止删除仍拥有项目的用户；演示出厂重置会清空其它用户、项目帧、标注、任务和私有模板，重新创建演示视频项目和一个已按文件名自然顺序生成帧的演示 DICOM 项目。
+5. 后端 `backend/routers/admin.py` 会阻止管理员删除、停用、改名或降级自己；项目库已共享，因此删除标注员不会删除或迁移项目；演示出厂重置会清空其它用户、项目帧、标注、任务和私有模板，重新创建演示视频项目和一个已按文件名自然顺序生成帧的演示 DICOM 项目。
 
 ### 项目与拆帧
 
diff --git a/doc/10-installation.md b/doc/10-installation.md
index c969082..fa217cc 100644
--- a/doc/10-installation.md
+++ b/doc/10-installation.md
@@ -184,6 +184,7 @@ minio_secure=false
 sam_default_model=sam2.1_hiera_tiny
 sam_model_path=/home/wkmgc/Desktop/Seg_Server/models/sam2.1_hiera_tiny.pt
 sam_model_config=configs/sam2.1/sam2.1_hiera_t.yaml
+sam3_external_enabled=false
 
 app_env=development
 cors_origins=["http://localhost:3000","http://127.0.0.1:3000"]