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/comparison_milestone.py

@@ -0,0 +1,36 @@
+"""比价战绩里程碑领取记录表。
+
+「记录比价战绩」每档(第 1~6 次)只能领一次,领取后写一行,(user_id, milestone)
+唯一,防止重复领奖。解锁进度由 comparison_record 里 status='success' 的条数决定,
+不存进度本身——只在这里记"哪几档已领"。仿 user_task 的一次性领取模型。
+"""
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Integer, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+
+
+class ComparisonMilestoneClaim(Base):
+    __tablename__ = "comparison_milestone_claim"
+    __table_args__ = (
+        UniqueConstraint("user_id", "milestone", name="uq_compare_milestone_user"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("user.id"), index=True, nullable=False
+    )
+    # 档位序号(1-based),见 app.core.rewards.RECORD_MILESTONES
+    milestone: Mapped[int] = mapped_column(Integer, nullable=False)
+    coin_awarded: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    claimed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    def __repr__(self) -> str:  # pragma: no cover
+        return f"<ComparisonMilestoneClaim user_id={self.user_id} m={self.milestone}>"