feat(custom-field): add join_description attribute for GDPR join-form labels

lib/membership/custom_field.ex

@@ -12,6 +12,8 @@ defmodule Mv.Membership.CustomField do
   - `slug` - URL-friendly, immutable identifier automatically generated from name (e.g., "phone-mobile")
   - `value_type` - Data type constraint (`:string`, `:integer`, `:boolean`, `:date`, `:email`). Immutable after creation.
   - `description` - Optional human-readable description
+  - `join_description` - Optional label shown for this field on the public join form
+    (e.g., a GDPR confirmation text); supports inline external links. Falls back to `name` when nil.
   - `required` - If true, all members must have this custom field (future feature)
   - `show_in_overview` - If true, this custom field will be displayed in the member overview table and can be sorted
 
@@ -61,7 +63,14 @@ defmodule Mv.Membership.CustomField do
   end
 
   actions do
-    default_accept [:name, :value_type, :description, :required, :show_in_overview]
+    default_accept [
+      :name,
+      :value_type,
+      :description,
+      :join_description,
+      :required,
+      :show_in_overview
+    ]
 
     read :read do
       primary? true
@@ -69,13 +78,13 @@ defmodule Mv.Membership.CustomField do
     end
 
     create :create do
-      accept [:name, :value_type, :description, :required, :show_in_overview]
+      accept [:name, :value_type, :description, :join_description, :required, :show_in_overview]
       change Mv.Membership.Changes.GenerateSlug
       validate string_length(:slug, min: 1)
     end
 
     update :update do
-      accept [:name, :description, :required, :show_in_overview]
+      accept [:name, :description, :join_description, :required, :show_in_overview]
       require_atomic? false
 
       validate fn changeset, _context ->
@@ -139,6 +148,15 @@ defmodule Mv.Membership.CustomField do
         trim?: true
       ]
 
+    attribute :join_description, :string,
+      allow_nil?: true,
+      public?: true,
+      description: "Label shown for this field on the public join form; supports external links",
+      constraints: [
+        max_length: 1000,
+        trim?: true
+      ]
+
     attribute :required, :boolean,
       default: false,
       allow_nil?: false