Remove legacy settings secret migration

- Remove the super-admin one-time migration from browser local settings into backend AI and speech settings.

- Delete the migration-specific SystemSettings unit test that no longer matches the desired behavior.

- Restore feature, module, testing, and progress docs so settings secrets are documented as backend-managed only.

docs/testing.md

@@ -90,7 +90,6 @@ AI 第三方接口、讯飞语音上游 WebSocket、麦克风权限和真实视
 | 后端模板兼容映射 | 已覆盖 | `server/src/templates/template.mapper.test.ts` |
 | 后端用户兼容映射 | 已覆盖 | `server/src/users/users.mapper.test.ts` |
 | 后端系统设置 schema | 已覆盖 | `server/src/settings/settings.schemas.test.ts` |
-| 系统设置旧密钥迁移 | 已覆盖 | `src/pages/SystemSettings.test.tsx` 覆盖后端空配置时从旧本地缓存合并 AI Key 和讯飞配置，并避免覆盖已有后端密钥。 |
 | 后端 AI 代理 schema | 已覆盖 | `server/src/ai/ai.schemas.test.ts` |
 | 后端语音代理首帧处理 | 已覆盖 | `server/src/speech/xf-frame.test.ts` |
 | 后端字段库 schema | 已覆盖 | `server/src/library/library.schemas.test.ts` |