feat(backend): add user custom attributes system

Add a flexible user attribute system that allows admins to define
custom fields for users (text, textarea, number, email, url, date,
select, multi_select types).

- Add Ent schemas for UserAttributeDefinition and UserAttributeValue
- Add service layer with validation logic
- Add repository layer with batch operations support
- Add admin API endpoints for CRUD and reorder operations
- Add batch API for loading attribute values for multiple users
- Add database migration (018_user_attributes.sql)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/ent/intercept/intercept.go

@@ -19,6 +19,8 @@ import (
 	"github.com/Wei-Shaw/sub2api/ent/usagelog"
 	"github.com/Wei-Shaw/sub2api/ent/user"
 	"github.com/Wei-Shaw/sub2api/ent/userallowedgroup"
+	"github.com/Wei-Shaw/sub2api/ent/userattributedefinition"
+	"github.com/Wei-Shaw/sub2api/ent/userattributevalue"
 	"github.com/Wei-Shaw/sub2api/ent/usersubscription"
 )
 
@@ -348,6 +350,60 @@ func (f TraverseUserAllowedGroup) Traverse(ctx context.Context, q ent.Query) err
 	return fmt.Errorf("unexpected query type %T. expect *ent.UserAllowedGroupQuery", q)
 }
 
+// The UserAttributeDefinitionFunc type is an adapter to allow the use of ordinary function as a Querier.
+type UserAttributeDefinitionFunc func(context.Context, *ent.UserAttributeDefinitionQuery) (ent.Value, error)
+
+// Query calls f(ctx, q).
+func (f UserAttributeDefinitionFunc) Query(ctx context.Context, q ent.Query) (ent.Value, error) {
+	if q, ok := q.(*ent.UserAttributeDefinitionQuery); ok {
+		return f(ctx, q)
+	}
+	return nil, fmt.Errorf("unexpected query type %T. expect *ent.UserAttributeDefinitionQuery", q)
+}
+
+// The TraverseUserAttributeDefinition type is an adapter to allow the use of ordinary function as Traverser.
+type TraverseUserAttributeDefinition func(context.Context, *ent.UserAttributeDefinitionQuery) error
+
+// Intercept is a dummy implementation of Intercept that returns the next Querier in the pipeline.
+func (f TraverseUserAttributeDefinition) Intercept(next ent.Querier) ent.Querier {
+	return next
+}
+
+// Traverse calls f(ctx, q).
+func (f TraverseUserAttributeDefinition) Traverse(ctx context.Context, q ent.Query) error {
+	if q, ok := q.(*ent.UserAttributeDefinitionQuery); ok {
+		return f(ctx, q)
+	}
+	return fmt.Errorf("unexpected query type %T. expect *ent.UserAttributeDefinitionQuery", q)
+}
+
+// The UserAttributeValueFunc type is an adapter to allow the use of ordinary function as a Querier.
+type UserAttributeValueFunc func(context.Context, *ent.UserAttributeValueQuery) (ent.Value, error)
+
+// Query calls f(ctx, q).
+func (f UserAttributeValueFunc) Query(ctx context.Context, q ent.Query) (ent.Value, error) {
+	if q, ok := q.(*ent.UserAttributeValueQuery); ok {
+		return f(ctx, q)
+	}
+	return nil, fmt.Errorf("unexpected query type %T. expect *ent.UserAttributeValueQuery", q)
+}
+
+// The TraverseUserAttributeValue type is an adapter to allow the use of ordinary function as Traverser.
+type TraverseUserAttributeValue func(context.Context, *ent.UserAttributeValueQuery) error
+
+// Intercept is a dummy implementation of Intercept that returns the next Querier in the pipeline.
+func (f TraverseUserAttributeValue) Intercept(next ent.Querier) ent.Querier {
+	return next
+}
+
+// Traverse calls f(ctx, q).
+func (f TraverseUserAttributeValue) Traverse(ctx context.Context, q ent.Query) error {
+	if q, ok := q.(*ent.UserAttributeValueQuery); ok {
+		return f(ctx, q)
+	}
+	return fmt.Errorf("unexpected query type %T. expect *ent.UserAttributeValueQuery", q)
+}
+
 // The UserSubscriptionFunc type is an adapter to allow the use of ordinary function as a Querier.
 type UserSubscriptionFunc func(context.Context, *ent.UserSubscriptionQuery) (ent.Value, error)
 
@@ -398,6 +454,10 @@ func NewQuery(q ent.Query) (Query, error) {
 		return &query[*ent.UserQuery, predicate.User, user.OrderOption]{typ: ent.TypeUser, tq: q}, nil
 	case *ent.UserAllowedGroupQuery:
 		return &query[*ent.UserAllowedGroupQuery, predicate.UserAllowedGroup, userallowedgroup.OrderOption]{typ: ent.TypeUserAllowedGroup, tq: q}, nil
+	case *ent.UserAttributeDefinitionQuery:
+		return &query[*ent.UserAttributeDefinitionQuery, predicate.UserAttributeDefinition, userattributedefinition.OrderOption]{typ: ent.TypeUserAttributeDefinition, tq: q}, nil
+	case *ent.UserAttributeValueQuery:
+		return &query[*ent.UserAttributeValueQuery, predicate.UserAttributeValue, userattributevalue.OrderOption]{typ: ent.TypeUserAttributeValue, tq: q}, nil
 	case *ent.UserSubscriptionQuery:
 		return &query[*ent.UserSubscriptionQuery, predicate.UserSubscription, usersubscription.OrderOption]{typ: ent.TypeUserSubscription, tq: q}, nil
 	default: