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.

backend/internal/provider/interface.go

@@ -0,0 +1,54 @@
+package provider
+
+import (
+	"context"
+)
+
+// ModelDiscovery 模型发现插件接口
+type ModelDiscovery interface {
+	// GetProviderID 返回服务商唯一标识
+	GetProviderID() string
+
+	// Match 判断是否匹配该服务商
+	// 基于 baseURL、headers 等信息判断
+	Match(baseURL string, headers map[string]string) bool
+
+	// DiscoverModels 发现模型列表
+	DiscoverModels(ctx context.Context, config DiscoveryConfig) ([]Model, error)
+
+	// GetMetadata 返回插件元数据
+	GetMetadata() ProviderMetadata
+}
+
+// DiscoveryConfig 模型发现配置
+type DiscoveryConfig struct {
+	BaseURL           string            // API 基础 URL
+	APIKey            string            // API 密钥
+	SecretKey         string            // 可选的密钥对
+	APIFormat         string            // API 格式：openai, gemini, anthropic
+	Headers           map[string]string // 自定义请求头
+	AllowLocalChannel bool              // 是否允许本地渠道
+	Region            string            // 地域（从 baseURL 解析或指定）
+}
+
+// Model 统一的模型定义
+type Model struct {
+	ID                     string         // 模型 ID
+	DisplayName            string         // 显示名称
+	Provider               string         // 提供商标识
+	Capability             []string       // 能力列表：text, image, video, audio, 3d
+	SupportedEndpointTypes []string       // 支持的端点类型
+	APIPath                string         // API 路径（如果非标准）
+	Metadata               map[string]any // 额外元数据
+	Deprecated             bool           // 是否已废弃
+	RequiresPlan           bool           // 是否需要特殊权限
+}
+
+// ProviderMetadata 插件元数据
+type ProviderMetadata struct {
+	Name             string   // 插件名称
+	Version          string   // 插件版本
+	Description      string   // 描述
+	Author           string   // 作者
+	SupportedRegions []string // 支持的地域
+}