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/service/wire.go

@@ -226,6 +226,7 @@ var ProviderSet = wire.NewSet(
 	ProvidePricingService,
 	NewBillingService,
 	NewBillingCacheService,
+	NewAnnouncementService,
 	NewAdminService,
 	NewGatewayService,
 	NewOpenAIGatewayService,