feat(provider): add plugin system for service-specific model discovery

Model discovery currently assumes every provider exposes all models via
GET {baseURL}/models. Providers like Alibaba Cloud Bailian serve video,
3D and some image models through dedicated endpoints, so 21 models across
5 families (HappyHorse / Wan / Kling / PixVerse / Vidu / Tripo) are never
discoverable - a functional blocker for a video creation workbench.

This introduces a ModelDiscovery plugin interface with a registry that
matches vendor plugins by base URL and falls back to the standard OpenAI
implementation, keeping core discovery logic vendor-agnostic.

- add provider.ModelDiscovery interface and plugin registry
- add openai plugin (standard behaviour, default fallback)
- add bailian plugin (standard + 21 extended multimodal models)
- gate behind ENABLE_PROVIDER_PLUGINS, default false
- register plugins lazily from service layer to avoid import cycle

Verified locally: flag off -> 238 models (unchanged); flag on -> 257
models (238 + 19 net new after dedup). OpenAI and Gemini channels
unaffected in both modes.

.env.example

@@ -8,6 +8,9 @@ CANVAS_ALLOWED_PRIVATE_UPSTREAM_HOSTS=
 CANVAS_DESKTOP_LOCAL_CHANNELS_ENABLED=false
 # 桌面本机渠道启用时必须显式使用：CANVAS_BACKEND_ADDR=127.0.0.1:8080
 CANVAS_CORS_ORIGINS=http://localhost:3000
+# 启用服务商插件系统以支持特定服务的模型发现（如阿里云百炼的视频生成模型）
+# 设置为 true 启用插件机制，false 使用传统方式（默认：false）
+ENABLE_PROVIDER_PLUGINS=false
 CANVAS_DATA_PATH=./data
 CANVAS_WORKER_CONCURRENCY=3
 CANVAS_CHANNEL_CONCURRENCY=3