feat(announcements): add admin/user announcement system

Implements announcements end-to-end (admin CRUD + read status, user list + mark read) with OR-of-AND targeting. Also breaks the ent<->service import cycle by moving schema-facing constants/targeting into a new domain package.

backend/internal/server/routes/user.go

@@ -64,6 +64,13 @@ func RegisterUserRoutes(
 			usage.POST("/dashboard/api-keys-usage", h.Usage.DashboardAPIKeysUsage)
 		}
 
+		// 公告（用户可见）
+		announcements := authenticated.Group("/announcements")
+		{
+			announcements.GET("", h.Announcement.List)
+			announcements.POST("/:id/read", h.Announcement.MarkRead)
+		}
+
 		// 卡密兑换
 		redeem := authenticated.Group("/redeem")
 		{