DoApiRequest previously called io.ReadAll on every request body to apply
passthrough model mapping, even when the feature was disabled. This
caused unnecessary memory pressure for large request bodies (long
conversations, multipart image uploads).

Split the guard logic into needsPassthroughModelMapping() so that
ReadAll is only called when all conditions are met (feature enabled +
passthrough mode + model mapped). Non-passthrough requests now pass
the original io.Reader directly to http.NewRequest without buffering.

Rewrite tests to verify combined guard + modify behavior.

- UserRatioSection 渠道 ID 输入改为带搜索的下拉选择器
- 使用日志详情、计费过程、展开行均显示用户倍率
- renderLogContent suffix 统一追加，消除 4 处重复
- 提取 dumpUpstreamRequest 辅助函数，debug 日志走 SysLog

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

- logger: 增加 ctx nil 检查，避免系统级日志 panic
- relay: 将 DebugEnabled 下的 println 替换为 SysLog
- price: 修复渠道定价在 ChannelMeta 未初始化时的获取失败问题

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