docs(admin): 补充管理后台 API 与数据库文档 (#12)

新增 admin 模块接口文档(auth login/me、admins CRUD、audit-logs、feedback 处理、stats 概览、user coins/detail/status、wallet 流水、withdraw 对账/刷新/列表) + admin_user/admin_audit_log 两张表文档，更新 api/database README 索引。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: OuYingJun1024 <1034284404@qq.com>
Reviewed-on: #12
Co-authored-by: ouzhou <ouzhou@wonderable.ai>
Co-committed-by: ouzhou <ouzhou@wonderable.ai>

app/api/v1/savings.py

@@ -42,6 +42,7 @@ def battle(user: CurrentUser, db: DbSession) -> SavingsBattleOut:
         week_saved_cents=b.week_saved_cents,
         beat_percent=b.beat_percent,
         streak_days=b.streak_days,
+        compare_count=b.compare_count,
     )