渠道支持多 API 格式 + 协议全名展示

- Channel 新增 formats(jsonb: chat|responses|messages), 一个渠道可同时声明
  支持 OpenAI Chat Completions / OpenAI Responses API / Anthropic Messages
- 路由改为按渠道声明的 formats 决定直通/转换: 客户端协议在 formats 内直通,
  否则转换为其首选支持格式(chat > messages > responses)
- 兼容旧数据: formats 为空时按 provider 推断(openai→chat+responses, anthropic→messages, compatible→chat)
- 前端: 渠道表/表单展示 API 格式(支持多选), usage 明细显示协议全名
  (OpenAI Chat Completions / OpenAI Responses API / Anthropic Messages)

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

server/internal/proxy/passthrough.go

@@ -53,7 +53,7 @@ func (g *Gateway) doProxy(c *gin.Context, cands []*store.Channel, clientProto st
 	var lastStatus = http.StatusBadGateway
 	var lastBody = []byte("all upstream channels failed")
 	for _, ch := range cands {
-		plan, err := prepareUpstream(ch.Provider, clientProto, body)
+		plan, err := prepareUpstream(ch, clientProto, body)
 		if err != nil {
 			lastStatus, lastBody = http.StatusInternalServerError, []byte("conversion error: "+err.Error())
 			continue