feat(savings): 省钱战绩卡比价口径 + 关停 savings demo 兜底

- 新增 GET /api/v1/compare/stats(comparison_record: status=success 计数 + saved_amount_cents 求和),供「我的」页省钱战绩卡比价口径(完成比价 + 累计发现可省),不要求下单
- savings.py 停用 demo seeder(ensure_seeded 等移除),只计真实 source='compare';简化 beat_percent 为 compare-only
- 测试: compare/stats 用例 + savings/welfare 改纯真实语义
- 文档: 新增 compare-stats.md;更新 savings-battle.md / savings_record.md / comparison_record.md / API README

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

app/api/v1/compare_record.py

@@ -20,6 +20,7 @@ from fastapi import APIRouter, HTTPException, Query, status
 from app.api.deps import CurrentUser, DbSession
 from app.repositories import comparison as crud_compare
 from app.schemas.compare_record import (
+    CompareStatsOut,
     ComparisonRecordCreatedOut,
     ComparisonRecordDetailOut,
     ComparisonRecordIn,
@@ -52,6 +53,16 @@ def report_record(
     return ComparisonRecordCreatedOut(id=rec.id)
 
 
+@router.get(
+    "/stats",
+    response_model=CompareStatsOut,
+    summary="比价口径战绩(完成比价数 + 累计发现可省)",
+)
+def stats(user: CurrentUser, db: DbSession) -> CompareStatsOut:
+    count, saved = crud_compare.get_stats(db, user.id)
+    return CompareStatsOut(compare_count=count, discovered_saved_cents=saved)
+
+
 @router.get(
     "/records",
     response_model=ComparisonRecordPage,