渠道: 模型名称映射(网关生效 + 渠道侧管理)

- 候选渠道携带 upstream_model, prepareUpstream 改写请求体 model 字段为上游名
- 新增渠道视角绑定 CRUD: GET/POST/PATCH/DELETE /admin/channels/:id/models
- 前端渠道页新增"模型映射": 列出绑定、内联改上游名、解除、添加
- rewriteModel 三种协议通用(model 均在顶层)

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

server/internal/proxy/passthrough.go

@@ -44,19 +44,28 @@ func parseBody(c *gin.Context) (*bodyReq, []byte, error) {
 	return br, body, nil
 }
 
-// upstreamURL 组装上游地址：base_url + 路径。
-// 兼容用户填完整 base（含 /v1）：去掉尾部 /v1，避免与请求路径重复。
+// upstreamURL 组装上游地址，智能识别用户填写的 Base URL 形式：
+//  - 完整端点(以目标资源路径结尾) → 直接使用
+//  - 已含版本前缀(如 /v1) → 只拼资源路径(/chat/completions 等)
+//  - 纯域名/地址前缀 → 拼完整路径(/v1/chat/completions 等)
 func upstreamURL(ch *store.Channel, path string) string {
 	base := strings.TrimRight(ch.BaseURL, "/")
-	return strings.TrimSuffix(base, "/v1") + path
+	if strings.HasSuffix(base, path) {
+		return base
+	}
+	if strings.HasSuffix(base, "/v1") {
+		return base + strings.TrimPrefix(path, "/v1")
+	}
+	return base + path
 }
 
 // doProxy 通用代理（M5）：遍历候选渠道，按需转换；可安全重试的失败自动故障转移。
-func (g *Gateway) doProxy(c *gin.Context, cands []*store.Channel, clientProto string, body []byte, stream bool, sink *usageSink) {
+func (g *Gateway) doProxy(c *gin.Context, cands []channel.Candidate, clientProto string, body []byte, stream bool, sink *usageSink) {
 	var lastStatus = http.StatusBadGateway
 	var lastBody = []byte("all upstream channels failed")
-	for _, ch := range cands {
-		plan, err := prepareUpstream(ch, clientProto, body)
+	for _, cand := range cands {
+		ch := cand.Channel
+		plan, err := prepareUpstream(ch, clientProto, body, cand.UpstreamModel)
 		if err != nil {
 			lastStatus, lastBody = http.StatusInternalServerError, []byte("conversion error: "+err.Error())
 			continue