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/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')

frontend/src/views/PlaygroundView.vue

@@ -116,9 +116,8 @@ const maxRefs = computed(() => {
     const extra = familyPreset.value ? 0 : maxVideos.value + maxAudios.value
     return Math.max(a, b) + extra
   }
-  // Image-to-image: honor the model's configured max (gpt-image-2=3,
-  // seedream-4.5=6, flux-klein-2=4 …). Fall back to 1 when image_to_image is on
-  // but no count was set.
+  // Image-to-image: honor the model's configured max (e.g. 3 or 6 refs, set per
+  // model). Fall back to 1 when image_to_image is on but no count was set.
   const m = Number(model.value?.max_reference_images || 0)
   if (m > 0) return m
   return model.value?.image_to_image ? 1 : 0

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