feat(admin): 渠道管理 - 模型供应商支持渠道级 SOCKS5/HTTP 代理

- ModelChannel 新增 proxy_url 字段，v48 迁移自动加列
- outbound 新增 OutboundHTTPClientWithProxy，socks5/socks5h/http/https 均支持；
  目标主机名不做本地解析，直接交给代理服务器远端解析，本地开启 TUN/VPN 时仍可用
- 系统代理、Provider 出站、管理员模型测试路径均使用渠道级代理
- 渠道管理表单新增"代理地址"输入框，留空回退全局代理

backend/internal/app/admin.go

@@ -93,6 +93,7 @@ type ChannelRequest struct {
 	Models               []string         `json:"models"`
 	Headers              []OutboundHeader `json:"headers"`
 	Enabled              *bool            `json:"enabled"`
+	ProxyURL             string           `json:"proxyUrl"`
 }
 
 type PublicModelChannel struct {
@@ -106,6 +107,7 @@ type PublicModelChannel struct {
 	APIKey           string                    `json:"apiKey"`
 	APIFormat        string                    `json:"apiFormat"`
 	ConcurrencyLimit int                       `json:"concurrencyLimit"`
+	ProxyURL         string                    `json:"proxyUrl"`
 	Models           []string                  `json:"models"`
 	ModelCosts       []PublicChannelModelPrice `json:"modelCosts"`
 	Headers          []OutboundHeader          `json:"headers,omitempty"`
@@ -187,6 +189,7 @@ func (s *Service) channelFromRequest(req ChannelRequest, channel model.ModelChan
 	}
 	channel.ModelsJSON = string(modelsJSON)
 	channel.HeadersJSON = headersJSON
+	channel.ProxyURL = strings.TrimSpace(req.ProxyURL)
 	if req.Enabled != nil {
 		channel.Enabled = *req.Enabled
 	}
@@ -206,6 +209,9 @@ func mergeChannelRequest(req ChannelRequest, channel model.ModelChannel) Channel
 	if req.Headers == nil {
 		req.Headers, _ = ParseOutboundHeadersJSON(channel.HeadersJSON)
 	}
+	if strings.TrimSpace(req.ProxyURL) == "" {
+		req.ProxyURL = channel.ProxyURL
+	}
 	return req
 }
 
@@ -255,6 +261,7 @@ func publicChannel(channel model.ModelChannel, admin bool, channelModels []model
 		APIKey:           apiKey,
 		APIFormat:        channel.APIFormat,
 		ConcurrencyLimit: channel.ConcurrencyLimit,
+		ProxyURL:         channel.ProxyURL,
 		Models:           models,
 		ModelCosts:       modelCosts,
 		Headers:          headers,