feat: add custom field slug

lib/membership/custom_field.ex

@@ -9,6 +9,7 @@ defmodule Mv.Membership.CustomField do
 
   ## Attributes
   - `name` - Unique identifier for the custom field (e.g., "phone_mobile", "birthday")
+  - `slug` - URL-friendly, immutable identifier automatically generated from name (e.g., "phone-mobile")
   - `value_type` - Data type constraint (`:string`, `:integer`, `:boolean`, `:date`, `:email`)
   - `description` - Optional human-readable description
   - `immutable` - If true, custom field values cannot be changed after creation
@@ -54,8 +55,14 @@ defmodule Mv.Membership.CustomField do
   end
 
   actions do
-    defaults [:create, :read, :update, :destroy]
+    defaults [:read, :update, :destroy]
     default_accept [:name, :value_type, :description, :immutable, :required]
+
+    create :create do
+      accept [:name, :value_type, :description, :immutable, :required]
+      change Mv.Membership.CustomField.Changes.GenerateSlug
+      validate string_length(:slug, min: 1)
+    end
   end
 
   attributes do
@@ -69,6 +76,15 @@ defmodule Mv.Membership.CustomField do
         trim?: true
       ]
 
+    attribute :slug, :string,
+      allow_nil?: false,
+      public?: true,
+      writable?: false,
+      constraints: [
+        max_length: 100,
+        trim?: true
+      ]
+
     attribute :value_type, :atom,
       constraints: [one_of: [:string, :integer, :boolean, :date, :email]],
       allow_nil?: false,
@@ -97,5 +113,6 @@ defmodule Mv.Membership.CustomField do
 
   identities do
     identity :unique_name, [:name]
+    identity :unique_slug, [:slug]
   end
 end