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>

docs/api/README.md

@@ -30,6 +30,7 @@
 | 12a | `POST /api/v1/compare/record` | Bearer | [详情](./compare-record-report.md) |
 | 12b | `GET /api/v1/compare/records` | Bearer | [详情](./compare-records.md) |
 | 12c | `GET /api/v1/compare/records/{id}` | Bearer | [详情](./compare-record-detail.md) |
+| 12e | `GET /api/v1/compare/stats` | Bearer | [详情](./compare-stats.md)（「我的」页省钱战绩卡:完成比价数 + 累计发现可省） |
 | **比价战绩里程碑**（前缀 `/api/v1/compare`；福利页「记录比价战绩」,按成功比价数解锁逐档发金币） |||
 | 12d | `GET /api/v1/compare/milestones` | Bearer | [详情](./compare-milestones.md) |
 | 12e | `POST /api/v1/compare/milestones/{milestone}/claim` | Bearer | [详情](./compare-milestone-claim.md) |