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/group.go

@@ -2,7 +2,7 @@ package schema
 
 import (
 	"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"
@@ -49,15 +49,15 @@ func (Group) Fields() []ent.Field {
 			Default(false),
 		field.String("status").
 			MaxLen(20).
-			Default(service.StatusActive),
+			Default(domain.StatusActive),
 
 		// Subscription-related fields (added by migration 003)
 		field.String("platform").
 			MaxLen(50).
-			Default(service.PlatformAnthropic),
+			Default(domain.PlatformAnthropic),
 		field.String("subscription_type").
 			MaxLen(20).
-			Default(service.SubscriptionTypeStandard),
+			Default(domain.SubscriptionTypeStandard),
 		field.Float("daily_limit_usd").
 			Optional().
 			Nillable().