feat!: 账号池改渠道网关,对外协议换成 vivid /v1

- 新增 channels/channel_models:上游 URL + AES-GCM 加密 key +
  并发/权重/本地统计,模型经渠道 /v1/models 导入绑定
- internal/upstream:vivid 协议客户端(NDJSON 流 + jobId 断流重连 +
  uploads 预签名 + models + balance + 错误分类)
- service/v1 重写:权重分组调度、Redis 并发槽 conc:c:、故障转移,
  站内/测试/对外统一走渠道执行器
- 对外端点换成 vivid 四件套:POST /v1/generate(NDJSON+jobId 重连)、
  POST /v1/uploads(SigV4 预签名 PUT)、GET /v1/models、GET /v1/balance,
  错误包 {"error":{"code","message"}}
- API key 新发改 vk_+48hex;删除全部内置 provider 与 token 池
- 管理端:渠道 CRUD/拉模型/导入/余额端点,前端账号页换渠道页,
  概览/文档/测试弹窗适配

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

frontend/src/views/SettingsView.vue

@@ -41,7 +41,7 @@ async function loadKey() {
 onMounted(loadKey)
 
 // Uniform mask for the hidden / persisted state — always "***" so the just-minted
-// preview and the server-stored preview read the same (no sk-xxx…yyyy mismatch).
+// preview and the server-stored preview read the same (no vk_xxx…yyyy mismatch).
 const maskedKey = computed(() => (apiKey.value || keyPreview.value) ? '***' : '')
 
 async function generateKey() {