refactor: 使用 go-sora2api SDK 替代自建 Sora 客户端

使用 go-sora2api v1.1.0 SDK 替代原有 ~2000 行自建 HTTP/PoW/TLS 指纹代码，
SDK 提供高并发性能优化（实例级 rand、PoW 缓冲区复用、context.Context 支持）。

- 新增 SoraSDKClient 适配器实现 SoraClient 接口
- 精简 sora_client.go 为仅保留接口和类型定义
- 更新 Wire 绑定使用 SoraSDKClient
- 删除 SoraDirectClient、sora_curl_cffi_sidecar、sora_request_guard 等旧代码

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

backend/internal/service/wire.go

@@ -206,14 +206,14 @@ func ProvideSoraMediaStorage(cfg *config.Config) *SoraMediaStorage {
 	return NewSoraMediaStorage(cfg)
 }
 
-func ProvideSoraDirectClient(
+func ProvideSoraSDKClient(
 	cfg *config.Config,
 	httpUpstream HTTPUpstream,
 	tokenProvider *OpenAITokenProvider,
 	accountRepo AccountRepository,
 	soraAccountRepo SoraAccountRepository,
-) *SoraDirectClient {
-	client := NewSoraDirectClient(cfg, httpUpstream, tokenProvider)
+) *SoraSDKClient {
+	client := NewSoraSDKClient(cfg, httpUpstream, tokenProvider)
 	client.SetAccountRepositories(accountRepo, soraAccountRepo)
 	return client
 }
@@ -306,8 +306,8 @@ var ProviderSet = wire.NewSet(
 	NewGatewayService,
 	ProvideSoraMediaStorage,
 	ProvideSoraMediaCleanupService,
-	ProvideSoraDirectClient,
-	wire.Bind(new(SoraClient), new(*SoraDirectClient)),
+	ProvideSoraSDKClient,
+	wire.Bind(new(SoraClient), new(*SoraSDKClient)),
 	NewSoraGatewayService,
 	NewOpenAIGatewayService,
 	NewOAuthService,