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/model/models.go

@@ -194,7 +194,7 @@ type Channel struct {
 	BaseURL   string `gorm:"size:500;not null"`
 	APIKeyEnc string `gorm:"type:text;not null"` // AES-GCM encrypted upstream API key
 	Enabled   bool   `gorm:"not null;default:true;index"`
-	// Concurrency caps simultaneous in-flight jobs on this channel. 0 = default.
+	// Concurrency caps simultaneous in-flight jobs on this channel. 0 = unlimited.
 	Concurrency int `gorm:"not null;default:0"`
 	// Weight biases weighted-random channel order: weight 2 is picked first at
 	// roughly 2x the rate of weight 1.