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/repository/wire.go

@@ -56,6 +56,8 @@ var ProviderSet = wire.NewSet(
 	NewProxyRepository,
 	NewRedeemCodeRepository,
 	NewPromoCodeRepository,
+	NewAnnouncementRepository,
+	NewAnnouncementReadRepository,
 	NewUsageLogRepository,
 	NewUsageCleanupRepository,
 	NewDashboardAggregationRepository,