feat(sora): 对齐 Sora OAuth 流程并隔离网关请求路径

- 新增并接通 Sora 专用 OAuth 接口与 ST/RT 换取能力
- 完成前端 Sora 授权、RT/ST 手动导入与账号创建流程
- 强化 Sora token 恢复、转发日志与网关路由隔离行为
- 补充后端服务层与路由层相关测试覆盖

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/sora_gateway_handler_test.go

@@ -43,6 +43,9 @@ func (s *stubSoraClient) CreateImageTask(ctx context.Context, account *service.A
 func (s *stubSoraClient) CreateVideoTask(ctx context.Context, account *service.Account, req service.SoraVideoRequest) (string, error) {
 	return "task-video", nil
 }
+func (s *stubSoraClient) EnhancePrompt(ctx context.Context, account *service.Account, prompt, expansionLevel string, durationS int) (string, error) {
+	return "enhanced prompt", nil
+}
 func (s *stubSoraClient) GetImageTask(ctx context.Context, account *service.Account, taskID string) (*service.SoraImageTaskStatus, error) {
 	return &service.SoraImageTaskStatus{ID: taskID, Status: "completed", URLs: s.imageURLs}, nil
 }