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/handler/handler.go

@@ -10,6 +10,7 @@ type AdminHandlers struct {
 	User             *admin.UserHandler
 	Group            *admin.GroupHandler
 	Account          *admin.AccountHandler
+	Announcement     *admin.AnnouncementHandler
 	OAuth            *admin.OAuthHandler
 	OpenAIOAuth      *admin.OpenAIOAuthHandler
 	GeminiOAuth      *admin.GeminiOAuthHandler
@@ -33,6 +34,7 @@ type Handlers struct {
 	Usage         *UsageHandler
 	Redeem        *RedeemHandler
 	Subscription  *SubscriptionHandler
+	Announcement  *AnnouncementHandler
 	Admin         *AdminHandlers
 	Gateway       *GatewayHandler
 	OpenAIGateway *OpenAIGatewayHandler