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/account.go

@@ -4,7 +4,7 @@ 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"
@@ -111,7 +111,7 @@ func (Account) Fields() []ent.Field {
 		// status: 账户状态，如 "active", "error", "disabled"
 		field.String("status").
 			MaxLen(20).
-			Default(service.StatusActive),
+			Default(domain.StatusActive),
 
 		// error_message: 错误信息，记录账户异常时的详细信息
 		field.String("error_message").