feat(compare): 比价记录落库 + 战绩里程碑(真发金币)

- comparison_record 表 + 上报(幂等 user+trace)/列表/详情 接口;服务端派生
  best/saved/is_source_best/status + information(done 帧文案/失败具体原因)
- 战绩里程碑:comparison_milestone_claim 表 + 进度/领取接口,按成功比价次数
  解锁,复用 grant_coins 真发金币(RECORD_MILESTONES 120/180/300/500/800/1200)
- 迁移 record/information/milestone 三个,单 head;rewards.py 集中档位配置
- 测试 test_compare_record(8)+test_compare_milestone(7) 全绿;docs/api 12a-12e + 索引

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

app/models/__init__.py

@@ -1,6 +1,8 @@
 """所有 ORM model 必须在这里 import 一次,Alembic / metadata 才能扫到。"""
 from app.models.ad_ecpm import AdEcpmRecord  # noqa: F401
 from app.models.ad_reward import AdRewardRecord  # noqa: F401
+from app.models.comparison import ComparisonRecord  # noqa: F401
+from app.models.comparison_milestone import ComparisonMilestoneClaim  # noqa: F401
 from app.models.feedback import Feedback  # noqa: F401
 from app.models.savings import SavingsRecord  # noqa: F401
 from app.models.signin import SigninRecord  # noqa: F401