feat: 公告系统 — Phase 2 完成

- Model/Repo/Service/Controller 全栈实现
- CRUD + ListActive（生效时间范围过滤）
- 管理后台独立页面 /admin/announcements（列表+弹窗CRUD）
- 前台首页 Hero 下方公告栏（info/success/warning/error 四色）
- Moderator+ 权限可管理公告

internal/router/frontend.go

@@ -33,11 +33,13 @@ func setupFrontendRoutes(r *gin.Engine, cfg *config.Config, d *dependencies) {
 					d.homeCache.Set(posts, total)
 				}
 			}
+			announcements, _ := d.announcementService.ListActive()
 			c.HTML(http.StatusOK, "home/index.html", common.BuildPageData(c, gin.H{
-				"Title":     "首页",
-				"ExtraCSS":  "/static/css/home.css",
-				"Posts":     posts,
-				"PostCount": total,
+				"Title":         "首页",
+				"ExtraCSS":      "/static/css/home.css",
+				"Posts":         posts,
+				"PostCount":     total,
+				"Announcements": announcements,
 			}))
 		})