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/repositories/savings.py

@@ -52,6 +52,7 @@ class SavingsBattle:
     week_saved_cents: int  # 本周(周一起)已省
     beat_percent: int  # 超过百分之多少用户
     streak_days: int  # 连续省钱天数
+    compare_count: int  # 累计完成比价次数(= 有效记录数:真实用 compare 记录、否则 demo 兜底)
 
 
 def _local_date(dt: datetime):
@@ -163,7 +164,10 @@ def get_battle(db: Session, user_id: int) -> SavingsBattle:
     beat_percent = _compute_beat_percent(db, user_id)
 
     return SavingsBattle(
-        week_saved_cents=week_saved, beat_percent=beat_percent, streak_days=streak
+        week_saved_cents=week_saved,
+        beat_percent=beat_percent,
+        streak_days=streak,
+        compare_count=len(records),
     )