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/controller/admin/announcement_controller.go

@@ -0,0 +1,102 @@
+package admin
+
+import (
+	"net/http"
+	"strconv"
+
+	"metazone.cc/mce/internal/common"
+	"metazone.cc/mce/internal/model"
+	"metazone.cc/mce/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+// AnnouncementController 公告管理控制器
+type AnnouncementController struct {
+	svc announcementUseCase
+}
+
+type announcementUseCase interface {
+	Create(a *model.Announcement) error
+	Update(a *model.Announcement) error
+	Delete(id uint) error
+	List(offset, limit int) ([]model.Announcement, int64, error)
+	ListActive() ([]model.Announcement, error)
+}
+
+// NewAnnouncementController 构造函数
+func NewAnnouncementController(svc announcementUseCase) *AnnouncementController {
+	return &AnnouncementController{svc: svc}
+}
+
+// Page 公告管理页面
+func (ac *AnnouncementController) Page(c *gin.Context) {
+	c.HTML(http.StatusOK, "admin/announcements/index.html", common.BuildAdminPageData(c, gin.H{
+		"Title":    "公告管理",
+		"ExtraCSS": "/admin/static/css/announcements.css",
+		"ExtraJS":  "/admin/static/js/announcements.js",
+		"Types":    service.AnnouncementTypes,
+	}))
+}
+
+// List 公告列表 API
+func (ac *AnnouncementController) List(c *gin.Context) {
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
+	offset := (page - 1) * pageSize
+
+	list, total, err := ac.svc.List(offset, pageSize)
+	if err != nil {
+		common.Error(c, http.StatusInternalServerError, "查询失败")
+		return
+	}
+	common.Ok(c, gin.H{"list": list, "total": total})
+}
+
+// Create 创建公告 API
+func (ac *AnnouncementController) Create(c *gin.Context) {
+	var req model.Announcement
+	if err := c.ShouldBindJSON(&req); err != nil {
+		common.Error(c, http.StatusBadRequest, "参数错误")
+		return
+	}
+	if err := ac.svc.Create(&req); err != nil {
+		common.Error(c, http.StatusInternalServerError, "创建失败")
+		return
+	}
+	common.OkMessage(c, "公告已创建")
+}
+
+// Update 更新公告 API
+func (ac *AnnouncementController) Update(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的 ID")
+		return
+	}
+	var req model.Announcement
+	if err := c.ShouldBindJSON(&req); err != nil {
+		common.Error(c, http.StatusBadRequest, "参数错误")
+		return
+	}
+	req.ID = uint(id)
+	if err := ac.svc.Update(&req); err != nil {
+		common.Error(c, http.StatusInternalServerError, "更新失败")
+		return
+	}
+	common.OkMessage(c, "公告已更新")
+}
+
+// Delete 删除公告 API
+func (ac *AnnouncementController) Delete(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的 ID")
+		return
+	}
+	if err := ac.svc.Delete(uint(id)); err != nil {
+		common.Error(c, http.StatusInternalServerError, "删除失败")
+		return
+	}
+	common.OkMessage(c, "公告已删除")
+}