feat(anthropic): 支持 API Key 自动透传并优化透传链路性能

- 新增 Anthropic API Key 自动透传开关与后端透传分支（仅替换认证）

- 账号编辑页新增自动透传开关，默认关闭

- 优化透传性能：SSE usage 解析 gjson 快路径、减少请求体重复拷贝、优化流式写回与非流式 usage 解析

- 补充单元测试与 benchmark，确保 Claude OAuth 路径不受影响

backend/internal/service/account_anthropic_passthrough_test.go

@@ -0,0 +1,62 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAccount_IsAnthropicAPIKeyPassthroughEnabled(t *testing.T) {
+	t.Run("Anthropic API Key 开启", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"anthropic_passthrough": true,
+			},
+		}
+		require.True(t, account.IsAnthropicAPIKeyPassthroughEnabled())
+	})
+
+	t.Run("Anthropic API Key 关闭", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"anthropic_passthrough": false,
+			},
+		}
+		require.False(t, account.IsAnthropicAPIKeyPassthroughEnabled())
+	})
+
+	t.Run("字段类型非法默认关闭", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"anthropic_passthrough": "true",
+			},
+		}
+		require.False(t, account.IsAnthropicAPIKeyPassthroughEnabled())
+	})
+
+	t.Run("非 Anthropic API Key 账号始终关闭", func(t *testing.T) {
+		oauth := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeOAuth,
+			Extra: map[string]any{
+				"anthropic_passthrough": true,
+			},
+		}
+		require.False(t, oauth.IsAnthropicAPIKeyPassthroughEnabled())
+
+		openai := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"anthropic_passthrough": true,
+			},
+		}
+		require.False(t, openai.IsAnthropicAPIKeyPassthroughEnabled())
+	})
+}