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_ws_client.go

@@ -11,6 +11,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	openaiwsv2 "github.com/Wei-Shaw/sub2api/internal/service/openai_ws_v2"
 	coderws "github.com/coder/websocket"
 	"github.com/coder/websocket/wsjson"
 )
@@ -234,6 +235,8 @@ type coderOpenAIWSClientConn struct {
 	conn *coderws.Conn
 }
 
+var _ openaiwsv2.FrameConn = (*coderOpenAIWSClientConn)(nil)
+
 func (c *coderOpenAIWSClientConn) WriteJSON(ctx context.Context, value any) error {
 	if c == nil || c.conn == nil {
 		return errOpenAIWSConnClosed
@@ -264,6 +267,30 @@ func (c *coderOpenAIWSClientConn) ReadMessage(ctx context.Context) ([]byte, erro
 	}
 }
 
+func (c *coderOpenAIWSClientConn) ReadFrame(ctx context.Context) (coderws.MessageType, []byte, error) {
+	if c == nil || c.conn == nil {
+		return coderws.MessageText, nil, errOpenAIWSConnClosed
+	}
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	msgType, payload, err := c.conn.Read(ctx)
+	if err != nil {
+		return coderws.MessageText, nil, err
+	}
+	return msgType, payload, nil
+}
+
+func (c *coderOpenAIWSClientConn) WriteFrame(ctx context.Context, msgType coderws.MessageType, payload []byte) error {
+	if c == nil || c.conn == nil {
+		return errOpenAIWSConnClosed
+	}
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	return c.conn.Write(ctx, msgType, payload)
+}
+
 func (c *coderOpenAIWSClientConn) Ping(ctx context.Context) error {
 	if c == nil || c.conn == nil {
 		return errOpenAIWSConnClosed