feat(openai-ws): 合并 WS v2 透传模式与前端 ws mode

新增 OpenAI WebSocket v2 passthrough relay 数据面与服务适配层，
支持按账号 ws mode 在 ctx_pool 与 passthrough 间路由。

同步调整前端 OpenAI ws mode 选项为 off/ctx_pool/passthrough，
并补充 i18n 文案与对应单测。

新增 Caddyfile.dmit 与 docker-compose-aicodex.yml 部署配置，
用于宿主机场景下的反向代理与服务编排。

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

backend/internal/service/openai_gateway_service.go

@@ -263,13 +263,15 @@ type OpenAIGatewayService struct {
 	toolCorrector       *CodexToolCorrector
 	openaiWSResolver    OpenAIWSProtocolResolver
 
-	openaiWSPoolOnce       sync.Once
-	openaiWSStateStoreOnce sync.Once
-	openaiSchedulerOnce    sync.Once
-	openaiWSPool           *openAIWSConnPool
-	openaiWSStateStore     OpenAIWSStateStore
-	openaiScheduler        OpenAIAccountScheduler
-	openaiAccountStats     *openAIAccountRuntimeStats
+	openaiWSPoolOnce              sync.Once
+	openaiWSStateStoreOnce        sync.Once
+	openaiSchedulerOnce           sync.Once
+	openaiWSPassthroughDialerOnce sync.Once
+	openaiWSPool                  *openAIWSConnPool
+	openaiWSStateStore            OpenAIWSStateStore
+	openaiScheduler               OpenAIAccountScheduler
+	openaiWSPassthroughDialer     openAIWSClientDialer
+	openaiAccountStats            *openAIAccountRuntimeStats
 
 	openaiWSFallbackUntil sync.Map // key: int64(accountID), value: time.Time
 	openaiWSRetryMetrics  openAIWSRetryMetrics