feat(billing): user billing ledger + summary endpoints
新增 GET /api/user/billing/ledger 与 /api/user/billing/summary 两个用户侧接口, 基于 Task11 的 list_user_ledger / summarize_user_ledger 返回当前用户的明细与聚合。 随文件一并提交此前累积的充值/支付/市场相关改动及测试辅助。 Co-Authored-By: Claude <noreply@anthropic.com>
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