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/frontend.go

@@ -34,12 +34,14 @@ func setupFrontendRoutes(r *gin.Engine, cfg *config.Config, d *dependencies) {
 				}
 			}
 			announcements, _ := d.announcementService.ListActive()
+			categoryNav, _ := d.categoryService.GetLeaves()
 			c.HTML(http.StatusOK, "home/index.html", common.BuildPageData(c, gin.H{
 				"Title":         "首页",
 				"ExtraCSS":      "/static/css/home.css",
 				"Posts":         posts,
 				"PostCount":     total,
 				"Announcements": announcements,
+				"CategoryNav":   categoryNav,
 			}))
 		})
 
@@ -56,6 +58,9 @@ func setupFrontendRoutes(r *gin.Engine, cfg *config.Config, d *dependencies) {
 		// 帖子页面
 		pages.GET("/posts", d.postCtrl.ListPage)
 		pages.GET("/posts/:id", d.postCtrl.ShowPage)
+
+		// 标签落地页
+		pages.GET("/tags/:slug", d.tagCtrl.TagPage)
 		pages.GET("/posts/:id/edit", d.authMdw.Required(), func(c *gin.Context) {
 			c.Redirect(http.StatusMovedPermanently, "/studio/write?id="+c.Param("id"))
 		})