feat(admin-brand): add question expansion service and related functionality

- Implemented QuestionExpansionService for generating and materializing questions based on combinations and AI distillation.
- Added question metadata classification logic to infer intent and layer of questions.
- Created API handlers for question expansion operations including combination preview, AI distillation, metadata classification, and materialization.
- Introduced database migrations to support new question-related fields and constraints in brand_questions and brand_keywords tables.
- Added caching mechanism for AI distillation results to improve performance.
- Defined JSON schemas for question distillation responses to ensure data integrity.

server/internal/shared/swagger/descriptions.go

@@ -175,10 +175,14 @@ var routeDocs = map[string]routeDoc{
 	"PUT /api/tenant/brands/:id/keywords/:kid":    {"更新关键词", "修改关键词文本/分组。"},
 	"DELETE /api/tenant/brands/:id/keywords/:kid": {"删除关键词", "删除某条关键词。"},
 
-	"GET /api/tenant/brands/:id/questions":         {"品牌问题列表", "返回品牌下的监控问题，可按 keyword_id 过滤。"},
-	"POST /api/tenant/brands/:id/questions":        {"新增监控问题", "为品牌添加一条 GEO 监控问题。"},
-	"PUT /api/tenant/brands/:id/questions/:qid":    {"更新监控问题", "修改问题文本或所属关键词。"},
-	"DELETE /api/tenant/brands/:id/questions/:qid": {"删除监控问题", "删除某条监控问题。"},
+	"GET /api/tenant/brands/:id/questions":                      {"品牌问题列表", "返回品牌下的监控问题，可按 keyword_id 过滤。"},
+	"POST /api/tenant/brands/:id/questions":                     {"新增监控问题", "为品牌添加一条 GEO 监控问题。"},
+	"POST /api/tenant/brands/:id/questions/combination-preview": {"拓词工具预览", "按地域词、前缀词、核心词、行业词、后缀词组合生成问题候选，仅预览不入库。"},
+	"POST /api/tenant/brands/:id/questions/ai-distill":          {"AI 扩展问题", "围绕品牌和主题生成问题候选，使用结构化输出并按 AI 点计费。"},
+	"POST /api/tenant/brands/:id/questions/classify-metadata":   {"问题元数据分类", "批量为问题文本推断 layer 和 intent 元数据。"},
+	"POST /api/tenant/brands/:id/questions/materialize":         {"保存问题候选", "将用户选中的问题候选保存到当前品牌问题集，执行配额、去重和审计。"},
+	"PUT /api/tenant/brands/:id/questions/:qid":                 {"更新监控问题", "修改问题文本或所属关键词。"},
+	"DELETE /api/tenant/brands/:id/questions/:qid":              {"删除监控问题", "删除某条监控问题。"},
 
 	"GET /api/tenant/brands/:id/competitors":         {"竞品列表", "返回品牌下登记的竞品。"},
 	"POST /api/tenant/brands/:id/competitors":        {"新增竞品", "为品牌添加竞品记录。"},