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

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

frontend/src/views/DocsView.vue

@@ -30,7 +30,7 @@ const videoModels = computed(() => models.value.filter((m) => m.type === 'video'
 function pubName(m) {
   return m?.alias || m?.id || ''
 }
-const sampleImage = computed(() => pubName(imageModels.value[0]) || 'gpt-image-2')
+const sampleImage = computed(() => pubName(imageModels.value[0]) || 'image-model')
 const sampleVideo = computed(() => pubName(videoModels.value[0]) || 'video-model')
 const sampleQuality = computed(() => (imageModels.value[0]?.resolutions || [])[0] || '1K')
 const sampleVQuality = computed(() => (videoModels.value[0]?.resolutions || [])[0] || '720p')