perf(service): SSE Scanner buffer 改用 sync.Pool 复用，减少高并发 GC 压力

将流式响应中 bufio.Scanner 的 64KB buffer 从每次 make 分配改为
sync.Pool 复用，统一切片表达式为 [:0]、变量命名为 scanBuf，
并补充对应的单元测试。

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

backend/internal/service/antigravity_gateway_service_test.go

@@ -7,7 +7,9 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
 	"github.com/gin-gonic/gin"
@@ -190,3 +192,37 @@ func TestAntigravityMaxRetriesForModel_AfterSwitchFallback(t *testing.T) {
 	got := antigravityMaxRetriesForModel("gemini-2.5-flash", true)
 	require.Equal(t, 5, got)
 }
+
+func TestAntigravityStreamUpstreamResponse_UsageAndFirstToken(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	writer := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(writer)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+	pr, pw := io.Pipe()
+	resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}, Body: pr}
+
+	go func() {
+		defer func() { _ = pw.Close() }()
+		_, _ = pw.Write([]byte("data: {\"usage\":{\"input_tokens\":1,\"output_tokens\":2,\"cache_read_input_tokens\":3,\"cache_creation_input_tokens\":4}}\n"))
+		_, _ = pw.Write([]byte("data: {\"usage\":{\"output_tokens\":5}}\n"))
+	}()
+
+	svc := &AntigravityGatewayService{}
+	start := time.Now().Add(-10 * time.Millisecond)
+	usage, firstTokenMs := svc.streamUpstreamResponse(c, resp, start)
+	_ = pr.Close()
+
+	require.NotNil(t, usage)
+	require.Equal(t, 1, usage.InputTokens)
+	// 第二次事件覆盖 output_tokens
+	require.Equal(t, 5, usage.OutputTokens)
+	require.Equal(t, 3, usage.CacheReadInputTokens)
+	require.Equal(t, 4, usage.CacheCreationInputTokens)
+
+	if firstTokenMs == nil {
+		t.Fatalf("expected firstTokenMs to be set")
+	}
+	// 确保有透传输出
+	require.True(t, strings.Contains(writer.Body.String(), "data:"))
+}