feat: 频道自动发现 — platform-bridge定时刷新频道路由 + ai-core周期同步

platform-bridge:
- QQ adapter: FetchGroups/FetchFriends 从NapCat API获取
- ChannelLister接口 + ChannelInfo结构体
- GET /api/v1/channels 端点 + cachedChannels缓存
- StartChannelRefresh 每10分钟自动刷新

ai-core:
- channel_sync.go: 每5分钟从platform-bridge拉取频道
- 自动更新thinker的platformChannels + botUIDs
- 不再依赖PLATFORM_CHANNELS环境变量手动配置

修复:
- SendMessage显式设AutoEscape:false
- PLATFORM_BRIDGE_URL默认端口8082→8095
- OBv11Params.AutoEscape去掉omitempty

Co-Authored-By: Claude <noreply@anthropic.com>

backend/platform-bridge/cmd/main.go

@@ -279,6 +279,7 @@ func main() {
 	mux := http.NewServeMux()
 	bh := handler.NewBridgeHandler(router)
 	bh.RegisterRoutes(mux)
+	bh.StartChannelRefresh(10 * time.Minute)
 
 	// Config and log handlers.
 	ch := handler.NewConfigHandler(configStore, router)