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/service/announcement_service.go

@@ -0,0 +1,59 @@
+package service
+
+import "metazone.cc/mce/internal/model"
+
+// AnnouncementTypes 公告类型及其显示名称和颜色
+var AnnouncementTypes = map[string]struct {
+	Label string
+	Color string
+}{
+	"info":    {"信息", "#3b82f6"},
+	"success": {"成功", "#22c55e"},
+	"warning": {"警告", "#f59e0b"},
+	"error":   {"错误", "#ef4444"},
+}
+
+// AnnouncementStore 公告仓库接口（ISP）
+type AnnouncementStore interface {
+	Create(a *model.Announcement) error
+	Update(a *model.Announcement) error
+	Delete(id uint) error
+	FindByID(id uint) (*model.Announcement, error)
+	List(offset, limit int) ([]model.Announcement, int64, error)
+	ListActive() ([]model.Announcement, error)
+}
+
+// AnnouncementService 公告业务逻辑
+type AnnouncementService struct {
+	repo AnnouncementStore
+}
+
+// NewAnnouncementService 构造函数
+func NewAnnouncementService(repo AnnouncementStore) *AnnouncementService {
+	return &AnnouncementService{repo: repo}
+}
+
+// Create 创建公告
+func (s *AnnouncementService) Create(a *model.Announcement) error {
+	return s.repo.Create(a)
+}
+
+// Update 更新公告
+func (s *AnnouncementService) Update(a *model.Announcement) error {
+	return s.repo.Update(a)
+}
+
+// Delete 删除公告
+func (s *AnnouncementService) Delete(id uint) error {
+	return s.repo.Delete(id)
+}
+
+// List 分页列表
+func (s *AnnouncementService) List(offset, limit int) ([]model.Announcement, int64, error) {
+	return s.repo.List(offset, limit)
+}
+
+// ListActive 生效中的公告
+func (s *AnnouncementService) ListActive() ([]model.Announcement, error) {
+	return s.repo.ListActive()
+}