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/repository/announcement_repo.go

@@ -0,0 +1,73 @@
+package repository
+
+import (
+	"time"
+
+	"metazone.cc/mce/internal/model"
+	"metazone.cc/mce/internal/service"
+
+	"gorm.io/gorm"
+)
+
+// AnnouncementRepo 公告数据访问
+type AnnouncementRepo struct {
+	db *gorm.DB
+}
+
+// NewAnnouncementRepo 构造函数
+func NewAnnouncementRepo(db *gorm.DB) *AnnouncementRepo {
+	return &AnnouncementRepo{db: db}
+}
+
+// WithTx 基于事务创建新实例
+func (r *AnnouncementRepo) WithTx(tx *gorm.DB) service.AnnouncementStore {
+	return &AnnouncementRepo{db: tx}
+}
+
+// Create 创建公告
+func (r *AnnouncementRepo) Create(a *model.Announcement) error {
+	return r.db.Create(a).Error
+}
+
+// Update 更新公告
+func (r *AnnouncementRepo) Update(a *model.Announcement) error {
+	return r.db.Save(a).Error
+}
+
+// Delete 删除公告
+func (r *AnnouncementRepo) Delete(id uint) error {
+	return r.db.Delete(&model.Announcement{}, id).Error
+}
+
+// FindByID 按 ID 查询
+func (r *AnnouncementRepo) FindByID(id uint) (*model.Announcement, error) {
+	var a model.Announcement
+	err := r.db.First(&a, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// List 分页查询全部公告（管理后台用）
+func (r *AnnouncementRepo) List(offset, limit int) ([]model.Announcement, int64, error) {
+	var total int64
+	if err := r.db.Model(&model.Announcement{}).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+	var list []model.Announcement
+	err := r.db.Order("sort ASC, created_at DESC").Offset(offset).Limit(limit).Find(&list).Error
+	return list, total, err
+}
+
+// ListActive 查询当前生效中的公告（前台用，按 sort 排序）
+func (r *AnnouncementRepo) ListActive() ([]model.Announcement, error) {
+	now := time.Now()
+	var list []model.Announcement
+	err := r.db.Where("is_active = true").
+		Where("start_at IS NULL OR start_at <= ?", now).
+		Where("end_at IS NULL OR end_at >= ?", now).
+		Order("sort ASC, created_at DESC").
+		Find(&list).Error
+	return list, err
+}