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/ent/schema/user_subscription.go

@@ -4,7 +4,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/ent/schema/mixins"
-	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/Wei-Shaw/sub2api/internal/domain"
 
 	"entgo.io/ent"
 	"entgo.io/ent/dialect"
@@ -44,7 +44,7 @@ func (UserSubscription) Fields() []ent.Field {
 			SchemaType(map[string]string{dialect.Postgres: "timestamptz"}),
 		field.String("status").
 			MaxLen(20).
-			Default(service.SubscriptionStatusActive),
+			Default(domain.SubscriptionStatusActive),
 
 		field.Time("daily_window_start").
 			Optional().