feat: add join form settings

lib/membership/membership.ex

@@ -455,6 +455,56 @@ defmodule Mv.Membership do
     end
   end
 
+  @doc """
+  Returns the allowlist of fields configured for the public join form.
+
+  Reads the current settings. When the join form is disabled (or no settings exist),
+  returns an empty list. When enabled, returns each configured field as a map with:
+    - `:id` - field identifier string (member field name or custom field UUID)
+    - `:required` - boolean; email is always true
+    - `:type` - `:member_field` or `:custom_field`
+
+  This is the server-side allowlist used by the join form submit action (Subtask 4)
+  to enforce which fields are accepted from user input.
+
+  ## Returns
+
+    - `[%{id: String.t(), required: boolean(), type: :member_field | :custom_field}]`
+    - `[]` when join form is disabled or settings are missing
+
+  ## Examples
+
+      iex> Mv.Membership.get_join_form_allowlist()
+      [%{id: "email", required: true, type: :member_field},
+       %{id: "first_name", required: false, type: :member_field}]
+
+  """
+  def get_join_form_allowlist do
+    case get_settings() do
+      {:ok, settings} ->
+        if settings.join_form_enabled do
+          build_join_form_allowlist(settings)
+        else
+          []
+        end
+
+      {:error, _} ->
+        []
+    end
+  end
+
+  defp build_join_form_allowlist(settings) do
+    field_ids = settings.join_form_field_ids || []
+    required_config = settings.join_form_field_required || %{}
+    member_field_names = Mv.Constants.member_fields() |> Enum.map(&Atom.to_string/1)
+
+    Enum.map(field_ids, fn id ->
+      type = if id in member_field_names, do: :member_field, else: :custom_field
+      required = Map.get(required_config, id, false)
+      %{id: id, required: required, type: type}
+    end)
+  end
+
   defp expired?(nil), do: true
   defp expired?(expires_at), do: DateTime.compare(expires_at, DateTime.utc_now()) == :lt
 end