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分组倍率和用户专属倍率在保存时没有校验，0 会触发计费层的 `<=0 → 1.0`
防御条款，结果订阅/余额分组按标准价扣费；完全是沉默地绕过了业务规则。

- 保存校验（admin_service）：CreateGroup / UpdateGroup / BatchSetGroupRateMultipliers /
  UpdateUser.SyncUserGroupRates 全部要求 > 0
- 计算层（billing_service）：三处 `<=0 → 1.0` 改为 `<0 → 0`；负数按 0 结算，
  避免配置异常被静默按 1x 收费
- 前端：分组倍率 / 用户专属倍率输入 min 统一到 0.001
- 删除未使用的 IsFreeSubscription 方法

测试：新增 billing_service_rate_multiplier_test.go 端到端验证；更新原有锁定
旧 `<=0 → 1.0` 行为的测试。

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints