feat(channel): 同步 vivid 最新渠道设计 — 手动全量同步端点/并发0=不限/同名别名报错

对齐 vivid 6039423/5802749/a1f808f/cfb6d49/aba8474/b01636f:
- 新增 POST /channels/sync-models:已保存渠道手动全量同步,上游不可达/
  空清单直接报错(保存路径仍降级为告警);渠道行按钮不再弹勾选框,
  表单内对已保存渠道同样直同步,新渠道保持预览+勾选即绑定
- 渠道并发 0 = 不限制(原 0=默认5),上限放开到 int32,槽位 Lua 本就
  max=0 放行
- 别名与上游模型同名提交即报错「留空即同名」,前后端双校验
- 渠道列表去掉上游地址与模型两列(同 vivid),状态徽章悬浮显示失败原因

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/internal/service/v1.go

@@ -64,8 +64,8 @@ const videoGenBudget = 30 * time.Minute
 // imageGenBudget caps one image generation end-to-end.
 const imageGenBudget = 8 * time.Minute
 
-// defaultChannelConcurrency applies when a channel's Concurrency is 0.
-const defaultChannelConcurrency = 5
+// Channel concurrency semantics: 0 = unlimited (the Lua slot script passes
+// max=0 straight through), matching vivid's channel design.
 
 // maxChannelFailover caps how many channels one request may burn on
 // retryable/temporary errors before giving up, so an upstream-wide blip can't
@@ -100,11 +100,11 @@ type V1Service struct {
 	conc *ConcurrencyService
 }
 
-// chanAcquire takes one per-channel slot (capped at max; 0 = default), tagged
+// chanAcquire takes one per-channel slot (capped at max; 0 = unlimited), tagged
 // with the generation's eventID.
 func (s *V1Service) chanAcquire(ctx context.Context, channelID, eventID string, max int) bool {
-	if max < 1 {
-		max = defaultChannelConcurrency
+	if max < 0 {
+		max = 0
 	}
 	return s.conc.Acquire(ctx, "conc:c:"+channelID, max, eventID)
 }