fix(sse): 修复非标准 SSE 格式解析问题

部分上游 API 返回的 SSE 格式不符合标准规范：
- 标准格式: `data: {...}`（冒号后有空格）
- 非标准格式: `data:{...}`（冒号后无空格）

使用预编译正则 `^data:\s*` 统一处理两种格式。

backend/internal/service/openai_gateway_service.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -28,6 +29,10 @@ const (
 	openaiStickySessionTTL = time.Hour // 粘性会话TTL
 )
 
+// openaiSSEDataRe matches SSE data lines with optional whitespace after colon.
+// Some upstream APIs return non-standard "data:" without space (should be "data: ").
+var openaiSSEDataRe = regexp.MustCompile(`^data:\s*`)
+
 // OpenAI allowed headers whitelist (for non-OAuth accounts)
 var openaiAllowedHeaders = map[string]bool{
 	"accept-language": true,
@@ -464,26 +469,33 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 	for scanner.Scan() {
 		line := scanner.Text()
 
-		// Replace model in response if needed
-		if needModelReplace && strings.HasPrefix(line, "data: ") {
-			line = s.replaceModelInSSELine(line, mappedModel, originalModel)
-		}
+		// Extract data from SSE line (supports both "data: " and "data:" formats)
+		if openaiSSEDataRe.MatchString(line) {
+			data := openaiSSEDataRe.ReplaceAllString(line, "")
 
-		// Forward line
-		if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
-			return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
-		}
-		flusher.Flush()
+			// Replace model in response if needed
+			if needModelReplace {
+				line = s.replaceModelInSSELine(line, mappedModel, originalModel)
+			}
+
+			// Forward line
+			if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+				return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+			}
+			flusher.Flush()
 
-		// Parse usage data
-		if strings.HasPrefix(line, "data: ") {
-			data := line[6:]
 			// Record first token time
 			if firstTokenMs == nil && data != "" && data != "[DONE]" {
 				ms := int(time.Since(startTime).Milliseconds())
 				firstTokenMs = &ms
 			}
 			s.parseSSEUsage(data, usage)
+		} else {
+			// Forward non-data lines as-is
+			if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+				return &openaiStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+			}
+			flusher.Flush()
 		}
 	}
 
@@ -495,7 +507,10 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 }
 
 func (s *OpenAIGatewayService) replaceModelInSSELine(line, fromModel, toModel string) string {
-	data := line[6:]
+	if !openaiSSEDataRe.MatchString(line) {
+		return line
+	}
+	data := openaiSSEDataRe.ReplaceAllString(line, "")
 	if data == "" || data == "[DONE]" {
 		return line
 	}