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/web/embed_on.go

@@ -86,6 +86,7 @@ func (s *FrontendServer) Middleware() gin.HandlerFunc {
 		if strings.HasPrefix(path, "/api/") ||
 			strings.HasPrefix(path, "/v1/") ||
 			strings.HasPrefix(path, "/v1beta/") ||
+			strings.HasPrefix(path, "/sora/") ||
 			strings.HasPrefix(path, "/antigravity/") ||
 			strings.HasPrefix(path, "/setup/") ||
 			path == "/health" ||
@@ -209,6 +210,7 @@ func ServeEmbeddedFrontend() gin.HandlerFunc {
 		if strings.HasPrefix(path, "/api/") ||
 			strings.HasPrefix(path, "/v1/") ||
 			strings.HasPrefix(path, "/v1beta/") ||
+			strings.HasPrefix(path, "/sora/") ||
 			strings.HasPrefix(path, "/antigravity/") ||
 			strings.HasPrefix(path, "/setup/") ||
 			path == "/health" ||