feat(server): KB prompt优化、字幕修复、S2S重连、助手配置API

- assistantProfileConfig: KB answer prompt改为分层策略(严格产品信息+灵活常识补充)
- nativeVoiceGateway: S2S upstream自动重连(最多50次)、event 351字幕debounce(800ms取最长文本)
- toolExecutor: 确定性query改写增强、KB查询传递session上下文
- contextKeywordTracker: 支持KB话题记忆优先enrichment
- contentSafeGuard: 新增品牌安全内容过滤服务
- assistantProfileService: 新增助手配置CRUD服务
- routes/assistantProfile: 新增助手配置API路由
- knowledgeKeywords: 扩展KB关键词词典
- fastAsrCorrector: ASR纠错规则更新
- tests/: KB prompt测试、保护窗口测试、Viking性能测试
- docs/: 助手配置API文档、系统提示词目录

test2/server/services/arkChatService.js

@@ -30,7 +30,7 @@ class ArkChatService {
     if (datasetIds.length === 0) return null;
 
     const topK = parseInt(process.env.VOLC_ARK_KNOWLEDGE_TOP_K) || 3;
-    const threshold = parseFloat(process.env.VOLC_ARK_KNOWLEDGE_THRESHOLD) || 0.5;
+    const threshold = parseFloat(process.env.VOLC_ARK_KNOWLEDGE_THRESHOLD) || 0.4;
 
     return {
       dataset_ids: datasetIds,