feat(admin): 设备存活监控接口 + 首次无障碍开启时间(first_protected_at) (#80)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---------

Co-authored-by: 陈世睿 <2839904623@qq.com>
Reviewed-on: #80
Co-authored-by: chenshirui <chenshirui@wonderable.ai>
Co-committed-by: chenshirui <chenshirui@wonderable.ai>

app/models/device.py

@@ -51,6 +51,11 @@ class DeviceLiveness(Base):
     ever_protected: Mapped[bool] = mapped_column(
         Boolean, nullable=False, default=False
     )
+    # 首次开无障碍(首次收到 accessibility_enabled 心跳)的时刻;ever_protected 第一次翻 true 时记一次,
+    # 后续心跳不覆盖。老设备(迁移前已 protected)无此值 → NULL。
+    first_protected_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
     # 最近一次 service 心跳时间(存活证明);超时即视为保护掉线
     last_heartbeat_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), index=True, nullable=True