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

templates/MetaLab-2026/static/css/posts.css

@@ -197,7 +197,72 @@
     gap: 12px;
 }
 
-/* ========== 404 ========== */
+/* ========== 赞/踩按钮 ========== */
+.post-reactions {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    margin-top: 20px;
+    padding: 12px 0;
+    border-top: 1px solid #e5e7eb;
+}
+
+.reaction-btn {
+    display: inline-flex;
+    align-items: center;
+    gap: 6px;
+    padding: 8px 16px;
+    border: 1px solid #e5e7eb;
+    border-radius: 8px;
+    background: #fff;
+    cursor: pointer;
+    font-size: 14px;
+    color: #6b7280;
+    transition: all 0.2s;
+    user-select: none;
+}
+
+.reaction-btn:hover {
+    border-color: #c4b5fd;
+    color: #6366f1;
+    background: #f5f3ff;
+}
+
+.reaction-btn:disabled {
+    opacity: 0.5;
+    cursor: not-allowed;
+}
+
+.reaction-btn.active.reaction-like-btn {
+    border-color: #6366f1;
+    color: #6366f1;
+    background: #f0eeff;
+}
+
+.reaction-btn.active.reaction-like-btn svg {
+    fill: #6366f1;
+    stroke: #6366f1;
+}
+
+.reaction-btn.active.reaction-dislike-btn {
+    border-color: #f87171;
+    color: #f87171;
+    background: #fef2f2;
+}
+
+.reaction-btn.active.reaction-dislike-btn svg {
+    fill: #f87171;
+    stroke: #f87171;
+}
+
+.reaction-like-count {
+    font-weight: 600;
+    min-width: 12px;
+}
+
+.reaction-btn.active .reaction-like-count {
+    color: inherit;
+}
 .error-404 {
     text-align: center;
     padding: 100px 0;