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/admin.go

@@ -29,6 +29,9 @@ func RegisterAdminRoutes(
 		// 账号管理
 		registerAccountRoutes(admin, h)
 
+		// 公告管理
+		registerAnnouncementRoutes(admin, h)
+
 		// OpenAI OAuth
 		registerOpenAIOAuthRoutes(admin, h)
 
@@ -229,6 +232,18 @@ func registerAccountRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 	}
 }
 
+func registerAnnouncementRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
+	announcements := admin.Group("/announcements")
+	{
+		announcements.GET("", h.Admin.Announcement.List)
+		announcements.POST("", h.Admin.Announcement.Create)
+		announcements.GET("/:id", h.Admin.Announcement.GetByID)
+		announcements.PUT("/:id", h.Admin.Announcement.Update)
+		announcements.DELETE("/:id", h.Admin.Announcement.Delete)
+		announcements.GET("/:id/read-status", h.Admin.Announcement.ListReadStatus)
+	}
+}
+
 func registerOpenAIOAuthRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 	openai := admin.Group("/openai")
 	{

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")
 		{