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/config/config_test.go

@@ -153,8 +153,8 @@ func TestLoadDefaultOpenAIWSConfig(t *testing.T) {
 	if cfg.Gateway.OpenAIWS.ModeRouterV2Enabled {
 		t.Fatalf("Gateway.OpenAIWS.ModeRouterV2Enabled = true, want false")
 	}
-	if cfg.Gateway.OpenAIWS.IngressModeDefault != "shared" {
-		t.Fatalf("Gateway.OpenAIWS.IngressModeDefault = %q, want %q", cfg.Gateway.OpenAIWS.IngressModeDefault, "shared")
+	if cfg.Gateway.OpenAIWS.IngressModeDefault != "ctx_pool" {
+		t.Fatalf("Gateway.OpenAIWS.IngressModeDefault = %q, want %q", cfg.Gateway.OpenAIWS.IngressModeDefault, "ctx_pool")
 	}
 }
 
@@ -1373,7 +1373,7 @@ func TestValidateConfig_OpenAIWSRules(t *testing.T) {
 			wantErr: "gateway.openai_ws.store_disabled_conn_mode",
 		},
 		{
-			name:    "ingress_mode_default 必须为 off|shared|dedicated",
+			name:    "ingress_mode_default 必须为 off|ctx_pool|passthrough",
 			mutate:  func(c *Config) { c.Gateway.OpenAIWS.IngressModeDefault = "invalid" },
 			wantErr: "gateway.openai_ws.ingress_mode_default",
 		},