feat: 实现赞/踩系统（P2）

- 新增 PostLike/PostDislike 模型，复合主键 (user_id, post_id)
- Post 表新增 LikesCount/DislikesCount 冗余计数器
- Repository 层：reaction_repo.go，含 WithTx 事务支持
- Service 层：reaction_service.go，赞踩互斥逻辑（6 态切换），事务包裹
- Controller 层：reaction_controller.go，3 个 API（ToggleLike/ToggleDislike/GetReaction）
- 接口定义：controller/interfaces.go 新增 reactionUseCase，service/repository.go 新增 ReactionStore
- 路由注册 + 依赖注入 + AutoMigrate
- 前端：文章详情页赞踩按钮 + 状态查询 + 401 跳转登录
- 样式：posts.css 新增赞踩按钮样式

cmd/server/main.go

@@ -27,7 +27,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("连接数据库失败: %v", err)
 	}
-	if err := db.AutoMigrate(&model.User{}, &model.AuditSubmission{}, &model.SiteSetting{}, &model.Notification{}, &model.Post{}, &model.UserCheckIn{}, &model.UserTask{}, &model.EnergyLog{}, &model.PostEnergizeLog{}, &model.DailyExpSummary{}, &model.Comment{}, &model.CommentMention{}, &model.CommunityFund{}, &model.FundLog{}); err != nil {
+	if err := db.AutoMigrate(&model.User{}, &model.AuditSubmission{}, &model.SiteSetting{}, &model.Notification{}, &model.Post{}, &model.UserCheckIn{}, &model.UserTask{}, &model.EnergyLog{}, &model.PostEnergizeLog{}, &model.DailyExpSummary{}, &model.Comment{}, &model.CommentMention{}, &model.CommunityFund{}, &model.FundLog{}, &model.PostLike{}, &model.PostDislike{}); err != nil {
 		log.Fatalf("数据库迁移失败: %v", err)
 	}