docs: 清理 OpenAI 时代残留,文档改述 vivid 协议 + 渠道架构

- README/README.en 重写:同构网关 + vivid 四端点 + 渠道同步/调度,
  明确不支持 OpenAI 协议上游;移除账号池/七大平台/OpenAI 兼容描述
- backend/frontend README、.env.example 更新(补 RUSTFS/COOKIE_SECURE/
  CHANNEL_SECRET)
- docker-compose 增加 CHANNEL_SECRET(渠道 key AES-GCM 密钥)
- 清掉代码注释/文档示例里的 OpenAI/gpt-image-2 残留

Generated with [Devin](https://devin.ai)

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

frontend/src/views/SettingsView.vue

@@ -26,7 +26,7 @@ onMounted(async () => {
   }
 })
 
-// ---- API Key (REAL: minted + verified server-side; OpenAI-compatible) ----
+// ---- API Key (REAL: minted + verified server-side; vivid /v1 protocol) ----
 // The full plaintext is returned only once, right after minting. On reload we
 // only have the server's masked preview (the plaintext is never stored).
 const apiKey = ref('')        // full plaintext — present only right after minting