feat(device): 无障碍保护存活心跳检测 + 掉线终端告警

- 新增 device 表 + /api/v1/device/{register,heartbeat} + 迁移 device_table
- heartbeat_monitor_worker 周期扫描心跳超时(App 被杀/无障碍停)→ 服务器终端打印告警
  (推送本期未接,先用 logger 终端打印代替真实通知;integrations/jpush.py 已备,后续直接替换)
- config / .env.example 增 JPUSH_* / HEARTBEAT_*
- 见 spec(仓库外 e:\codes\spec\accessibility-liveness-push.md)

注:本提交同时快照了并行 session 未提交的 coupon_state 改动 + 相关 migration
(与本功能同迁移链耦合,无法单独拆分,经确认一并提交)。

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

app/models/__init__.py

@@ -7,6 +7,7 @@ from app.models.admin import AdminAuditLog, AdminUser  # noqa: F401
 from app.models.app_config import AppConfig  # noqa: F401
 from app.models.comparison import ComparisonRecord  # noqa: F401
 from app.models.comparison_milestone import ComparisonMilestoneClaim  # noqa: F401
+from app.models.device import Device  # noqa: F401
 from app.models.coupon_state import (  # noqa: F401
     CouponClaimRecord,
     CouponDailyCompletion,