feat(coupon): 领券今日状态后端(弹窗频控 + 领券记录)

- 两张表 + Alembic 迁移:coupon_prompt_engagement(device×日,弹窗频控源)/ coupon_claim_record(device×券×日,领券记录·资产)
- coupon/step 透传顺手写库:step0 写 engagement(claim_started);响应解析 last_coupon_result/coupon_results 幂等写 claim record(按 coupon_id 去重,避免 done 帧 last 与 coupon_results 重复在 autoflush=False 下撞唯一约束回滚整批)
- 新增 GET /coupon/prompt/should-show(弹窗判据)+ POST /coupon/prompt/dismiss(拒绝通知,透传链路看不到拒绝)
- 判断按 device_id(券发到设备登录的外卖账号,比 user 更贴近登录环境);user_id 仅资产留痕、可空
- MVP 先不去重(队列照旧全发);pricebot 未改

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

app/models/__init__.py

@@ -7,6 +7,10 @@ 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.coupon_state import (  # noqa: F401
+    CouponClaimRecord,
+    CouponPromptEngagement,
+)
 from app.models.feedback import Feedback  # noqa: F401
 from app.models.meituan_coupon import MeituanCoupon  # noqa: F401
 from app.models.ops_marquee_seed import OpsMarqueeSeed  # noqa: F401