feat: 分类系统 + 标签系统 — Phase 4 完成

分类：Model/Repo/Service/Controller + 管理后台树形页面 + 首页导航栏
标签：Model/Repo/Service/Controller + /tags/{slug} 落地页 + 写文章页输入
Post 加 CategoryID，Create/Update 支持分类和标签
SQL 迁移含 categories/tags/post_tags 表及预设未分类

internal/router/deps_core.go

@@ -47,6 +47,10 @@ type dependencies struct {
 	homeCache         *cache.HomePageCache
 	announcementService *service.AnnouncementService
 	announcementCtrl    *adminCtrl.AnnouncementController
+	categoryCtrl        *adminCtrl.CategoryController
+	categoryService     *service.CategoryService
+	tagCtrl             *controller.TagController
+	tagService          *service.TagService
 }
 
 func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings, redisClient *redis.Client) (