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/redeem_code.go

@@ -3,7 +3,7 @@ package schema
 import (
 	"time"
 
-	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/Wei-Shaw/sub2api/internal/domain"
 
 	"entgo.io/ent"
 	"entgo.io/ent/dialect"
@@ -41,13 +41,13 @@ func (RedeemCode) Fields() []ent.Field {
 			Unique(),
 		field.String("type").
 			MaxLen(20).
-			Default(service.RedeemTypeBalance),
+			Default(domain.RedeemTypeBalance),
 		field.Float("value").
 			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
 			Default(0),
 		field.String("status").
 			MaxLen(20).
-			Default(service.StatusUnused),
+			Default(domain.StatusUnused),
 		field.Int64("used_by").
 			Optional().
 			Nillable(),