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/migrate/schema.go

@@ -477,7 +477,6 @@ var (
 		{Name: "concurrency", Type: field.TypeInt, Default: 5},
 		{Name: "status", Type: field.TypeString, Size: 20, Default: "active"},
 		{Name: "username", Type: field.TypeString, Size: 100, Default: ""},
-		{Name: "wechat", Type: field.TypeString, Size: 100, Default: ""},
 		{Name: "notes", Type: field.TypeString, Default: "", SchemaType: map[string]string{"postgres": "text"}},
 	}
 	// UsersTable holds the schema information for the "users" table.
@@ -531,6 +530,92 @@ var (
 			},
 		},
 	}
+	// UserAttributeDefinitionsColumns holds the columns for the "user_attribute_definitions" table.
+	UserAttributeDefinitionsColumns = []*schema.Column{
+		{Name: "id", Type: field.TypeInt64, Increment: true},
+		{Name: "created_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
+		{Name: "updated_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
+		{Name: "deleted_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
+		{Name: "key", Type: field.TypeString, Size: 100},
+		{Name: "name", Type: field.TypeString, Size: 255},
+		{Name: "description", Type: field.TypeString, Default: "", SchemaType: map[string]string{"postgres": "text"}},
+		{Name: "type", Type: field.TypeString, Size: 20},
+		{Name: "options", Type: field.TypeJSON, SchemaType: map[string]string{"postgres": "jsonb"}},
+		{Name: "required", Type: field.TypeBool, Default: false},
+		{Name: "validation", Type: field.TypeJSON, SchemaType: map[string]string{"postgres": "jsonb"}},
+		{Name: "placeholder", Type: field.TypeString, Size: 255, Default: ""},
+		{Name: "display_order", Type: field.TypeInt, Default: 0},
+		{Name: "enabled", Type: field.TypeBool, Default: true},
+	}
+	// UserAttributeDefinitionsTable holds the schema information for the "user_attribute_definitions" table.
+	UserAttributeDefinitionsTable = &schema.Table{
+		Name:       "user_attribute_definitions",
+		Columns:    UserAttributeDefinitionsColumns,
+		PrimaryKey: []*schema.Column{UserAttributeDefinitionsColumns[0]},
+		Indexes: []*schema.Index{
+			{
+				Name:    "userattributedefinition_key",
+				Unique:  false,
+				Columns: []*schema.Column{UserAttributeDefinitionsColumns[4]},
+			},
+			{
+				Name:    "userattributedefinition_enabled",
+				Unique:  false,
+				Columns: []*schema.Column{UserAttributeDefinitionsColumns[13]},
+			},
+			{
+				Name:    "userattributedefinition_display_order",
+				Unique:  false,
+				Columns: []*schema.Column{UserAttributeDefinitionsColumns[12]},
+			},
+			{
+				Name:    "userattributedefinition_deleted_at",
+				Unique:  false,
+				Columns: []*schema.Column{UserAttributeDefinitionsColumns[3]},
+			},
+		},
+	}
+	// UserAttributeValuesColumns holds the columns for the "user_attribute_values" table.
+	UserAttributeValuesColumns = []*schema.Column{
+		{Name: "id", Type: field.TypeInt64, Increment: true},
+		{Name: "created_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
+		{Name: "updated_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
+		{Name: "value", Type: field.TypeString, Size: 2147483647, Default: ""},
+		{Name: "user_id", Type: field.TypeInt64},
+		{Name: "attribute_id", Type: field.TypeInt64},
+	}
+	// UserAttributeValuesTable holds the schema information for the "user_attribute_values" table.
+	UserAttributeValuesTable = &schema.Table{
+		Name:       "user_attribute_values",
+		Columns:    UserAttributeValuesColumns,
+		PrimaryKey: []*schema.Column{UserAttributeValuesColumns[0]},
+		ForeignKeys: []*schema.ForeignKey{
+			{
+				Symbol:     "user_attribute_values_users_attribute_values",
+				Columns:    []*schema.Column{UserAttributeValuesColumns[4]},
+				RefColumns: []*schema.Column{UsersColumns[0]},
+				OnDelete:   schema.NoAction,
+			},
+			{
+				Symbol:     "user_attribute_values_user_attribute_definitions_values",
+				Columns:    []*schema.Column{UserAttributeValuesColumns[5]},
+				RefColumns: []*schema.Column{UserAttributeDefinitionsColumns[0]},
+				OnDelete:   schema.NoAction,
+			},
+		},
+		Indexes: []*schema.Index{
+			{
+				Name:    "userattributevalue_user_id_attribute_id",
+				Unique:  true,
+				Columns: []*schema.Column{UserAttributeValuesColumns[4], UserAttributeValuesColumns[5]},
+			},
+			{
+				Name:    "userattributevalue_attribute_id",
+				Unique:  false,
+				Columns: []*schema.Column{UserAttributeValuesColumns[5]},
+			},
+		},
+	}
 	// UserSubscriptionsColumns holds the columns for the "user_subscriptions" table.
 	UserSubscriptionsColumns = []*schema.Column{
 		{Name: "id", Type: field.TypeInt64, Increment: true},
@@ -627,6 +712,8 @@ var (
 		UsageLogsTable,
 		UsersTable,
 		UserAllowedGroupsTable,
+		UserAttributeDefinitionsTable,
+		UserAttributeValuesTable,
 		UserSubscriptionsTable,
 	}
 )
@@ -676,6 +763,14 @@ func init() {
 	UserAllowedGroupsTable.Annotation = &entsql.Annotation{
 		Table: "user_allowed_groups",
 	}
+	UserAttributeDefinitionsTable.Annotation = &entsql.Annotation{
+		Table: "user_attribute_definitions",
+	}
+	UserAttributeValuesTable.ForeignKeys[0].RefTable = UsersTable
+	UserAttributeValuesTable.ForeignKeys[1].RefTable = UserAttributeDefinitionsTable
+	UserAttributeValuesTable.Annotation = &entsql.Annotation{
+		Table: "user_attribute_values",
+	}
 	UserSubscriptionsTable.ForeignKeys[0].RefTable = GroupsTable
 	UserSubscriptionsTable.ForeignKeys[1].RefTable = UsersTable
 	UserSubscriptionsTable.ForeignKeys[2].RefTable = UsersTable