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>

backend/.env.example

@@ -1,8 +1,8 @@
-# Vivid AI backend config template. Copy to .env and fill in real values.
+# image2api backend config template. Copy to .env and fill in real values.
 # Real environment variables override .env.
 
 APP_ENV=development
-APP_TITLE=Vivid AI
+APP_TITLE=image2api
 HTTP_ADDR=:6666
 
 # PostgreSQL (database must already exist; tables auto-migrate on boot)
@@ -13,8 +13,22 @@ REDIS_ADDR=127.0.0.1:6379
 REDIS_PASSWORD=
 REDIS_DB=0
 
+# RustFS / S3-compatible object storage (generated media + reference uploads)
+RUSTFS_ENDPOINT=http://127.0.0.1:9000
+RUSTFS_BUCKET=vivid-ai
+RUSTFS_ACCESS_KEY=YOUR_AK
+RUSTFS_SECRET_KEY=YOUR_SK
+
+# Channel upstream API keys are stored AES-GCM encrypted with this secret.
+# Set a random value in production and keep it stable — changing it makes
+# previously saved channel keys undecryptable.
+CHANNEL_SECRET=change-me-channel-secret
+
 # Frontend dev origins allowed for CORS
 CORS_ORIGINS=http://localhost:5173,http://127.0.0.1:5173
 
-# Optional: override generated media root (defaults to ../../ai-gateway/data/generated)
-# GENERATED_ROOT=
+# Set to true when serving the site over HTTPS (session cookie Secure flag)
+COOKIE_SECURE=false
+
+# SESSION_COOKIE_NAME=vivid_session
+# GENERATED_ROOT=   # optional: override generated media root