fix: 修复 Chat Completions 编译错误和运行时 panic

1. 修复 WriteFilteredHeaders API 不兼容（2处）:
   将 s.cfg.Security.ResponseHeaders 改为 s.responseHeaderFilter，
   因为 main 分支已将函数签名改为接受 *responseheaders.CompiledHeaderFilter

2. 修复 writer 生命周期导致的 nil pointer panic:
   ChatCompletions handler 替换了 c.Writer 但未恢复，导致
   OpsErrorLogger 中间件的 defer 释放 opsCaptureWriter 后，
   Logger 中间件调用 c.Writer.Status() 触发空指针解引用。
   通过保存并恢复 originalWriter 修复。

3. 为 chatCompletionsResponseWriter 添加防御性 Status() 和
   Written() 方法，包含 nil 安全检查

4. 恢复 gateway.go 中被误删的 net/http import

backend/internal/handler/openai_chat_completions.go

@@ -62,6 +62,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 
 	stream, _ := converted["stream"].(bool)
 	model, _ := converted["model"].(string)
+	originalWriter := c.Writer
 	writer := newChatCompletionsResponseWriter(c.Writer, stream, includeUsage, model)
 	c.Writer = writer
 	c.Request.Body = io.NopCloser(bytes.NewReader(convertedBody))
@@ -69,6 +70,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 
 	h.Responses(c)
 	writer.Finalize()
+	c.Writer = originalWriter
 }
 
 type chatCompletionsResponseWriter struct {
@@ -167,6 +169,20 @@ func (w *chatCompletionsResponseWriter) SetPassthrough() {
 	w.passthrough = true
 }
 
+func (w *chatCompletionsResponseWriter) Status() int {
+	if w.ResponseWriter == nil {
+		return 0
+	}
+	return w.ResponseWriter.Status()
+}
+
+func (w *chatCompletionsResponseWriter) Written() bool {
+	if w.ResponseWriter == nil {
+		return false
+	}
+	return w.ResponseWriter.Written()
+}
+
 func (w *chatCompletionsResponseWriter) flushStreamBuffer() {
 	for {
 		buf := w.streamBuf.Bytes()

backend/internal/server/routes/gateway.go

@@ -1,6 +1,8 @@
 package routes
 
 import (
+	"net/http"
+
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler"
 	"github.com/Wei-Shaw/sub2api/internal/server/middleware"

backend/internal/service/openai_chat_completions_forward.go

@@ -209,8 +209,8 @@ func (s *OpenAIGatewayService) buildChatCompletionsRequest(ctx context.Context,
 }
 
 func (s *OpenAIGatewayService) handleChatCompletionsStreamingResponse(ctx context.Context, resp *http.Response, c *gin.Context, account *Account, startTime time.Time, originalModel, mappedModel string) (*chatStreamingResult, error) {
-	if s.cfg != nil {
+	if s.responseHeaderFilter != nil {
-		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.cfg.Security.ResponseHeaders)
+		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
 	}
 
 	c.Header("Content-Type", "text/event-stream")
@@ -409,7 +409,9 @@ func (s *OpenAIGatewayService) handleChatCompletionsNonStreamingResponse(resp *h
 	}
 	body = s.correctToolCallsInResponseBody(body)
 
-	responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.cfg.Security.ResponseHeaders)
+	if s.responseHeaderFilter != nil {
+		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
+	}
 
 	contentType := "application/json"
 	if s.cfg != nil && !s.cfg.Security.ResponseHeaders.Enabled {