feat(onboarding): 新手引导完成按 设备+账号 去重(表/模型/仓储/端点/迁移/测试)

- onboarding_completion 表(user_id+device_id 唯一约束)+ model + repository
- POST /api/v1/user/onboarding/complete 标记完成;登录响应 TokenWithUser.onboarding_completed
  (按登录请求带的 device_id 判定是否已走过引导,跨卸载重装稳定)
- alembic 迁移 onboarding_completion(rebase 到最新 main 后已链在 coupon_daily_completion 之后,单 head)
- docs/database 文档 + tests/test_onboarding.py + run.bat(本地启动脚本)

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

app/models/__init__.py

@@ -16,6 +16,7 @@ from app.models.feedback import Feedback  # noqa: F401
 from app.models.invite import InviteRelation  # noqa: F401
 from app.models.invite_fingerprint import InviteFingerprint  # noqa: F401
 from app.models.meituan_coupon import MeituanCoupon  # noqa: F401
+from app.models.onboarding import OnboardingCompletion  # noqa: F401
 from app.models.ops_marquee_seed import OpsMarqueeSeed  # noqa: F401
 from app.models.ops_stat_config import OpsStatConfig  # noqa: F401
 from app.models.price_observation import PriceObservation  # noqa: F401