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>

backend/internal/repo/event_repo.go

@@ -184,7 +184,7 @@ type DashboardWindow struct {
 	Pending int64   `json:"pending"`
 	Image   int64   `json:"image"`
 	Video   int64   `json:"video"`
-	API     int64   `json:"api"` // source = 'v1' (OpenAI-compatible key)
+	API     int64   `json:"api"` // source = 'v1' (vk_ external API key)
 	Web     int64   `json:"web"` // everything else (web / playground)
 	Spent   float64 `json:"spent"`
 }

backend/internal/service/admin_write.go

@@ -653,8 +653,8 @@ func boolValueWithDefault(v any, fallback bool) bool {
 
 // resolutionsFromPrices mirrors Python models_store._resolutions_from_prices:
 // an image model's quality tiers ARE its price keys (the admin form never sends
-// `resolutions` for images), returned in canonical 1K/2K/4K order. gpt-image-2,
-// for example, only ever has a "1K" price, so it resolves to exactly ["1K"].
+// `resolutions` for images), returned in canonical 1K/2K/4K order. A model that
+// only ever has a "1K" price, for example, resolves to exactly ["1K"].
 func resolutionsFromPrices(prices datatypes.JSONMap) datatypes.JSON {
 	out := []string{}
 	for _, r := range []string{"1K", "2K", "4K"} {