feat: 更新数据库时支持同步保存子表配置和DDL

- UpdateDatabaseRequest 添加 SubTables 字段
- 数据库更新时同步创建或更新子表记录
- 支持子表 DDL 的保存

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/internal/service/sub_table_service.go

@@ -119,6 +119,7 @@ func (s *SubTableService) Create(req model.CreateSubTableRequest) (*model.SubTab
 		MappingType:     req.MappingType,
 		RelationField:   req.RelationField,
 		RelationType:    req.RelationType,
+		DDL:            req.DDL,
 	}
 
 	if err := s.repo.Create(info); err != nil {
@@ -196,6 +197,8 @@ func (s *SubTableService) Update(id string, req model.UpdateSubTableRequest) (*m
 	if req.RelationType != "" {
 		info.RelationType = req.RelationType
 	}
+	// 更新 DDL
+	info.DDL = req.DDL
 
 	if err := s.repo.Update(id, info); err != nil {
 		log.Printf("[SubTable Update] 更新失败: %v", err)