feat: SearXNG 搜索集成 + DevTools Docker + PG 备份 + 文档更新

- web_search 工具/插件接入自托管 SearXNG，支持百度/必应/搜狗/360搜索
- DevTools 加入 docker-compose.dev.yml，devtools/Dockerfile
- scripts/pg-backup.sh 数据库备份恢复脚本，docs/pg-backup-migration.md
- 后台思考 + datetime 插件时区默认 Asia/Shanghai
- docker-compose 对齐 volume 名称，清理 tool-engine 残留引用
- README.md / Deploy.md 更新至当前架构（移除简报/tool-engine，新增搜索/跨端同步/DevTools）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/ai-core/internal/tools/datetime_tool.go

@@ -303,11 +303,15 @@ func (t *DateTimeTool) handleTimezoneList() (*ToolResult, error) {
 	}, nil
 }
 
-// getTimezone extracts the timezone from arguments, defaulting to local.
+// getTimezone extracts the timezone from arguments, defaulting to Asia/Shanghai.
 func (t *DateTimeTool) getTimezone(arguments map[string]interface{}) (*time.Location, error) {
 	tzName, _ := arguments["timezone"].(string)
 	if tzName == "" {
-		return time.Local, nil
+		loc, err := time.LoadLocation("Asia/Shanghai")
+		if err != nil {
+			return time.Local, nil
+		}
+		return loc, nil
 	}
 	loc, err := time.LoadLocation(tzName)
 	if err != nil {