chore: adds settings ressource and migration

lib/membership/membership.ex

@@ -6,12 +6,14 @@ defmodule Mv.Membership do
   - `Member` - Club members with personal information and custom field values
   - `CustomFieldValue` - Dynamic custom field values attached to members
   - `CustomField` - Schema definitions for custom fields
+  - `Setting` - Global application settings (singleton)
 
   ## Public API
   The domain exposes these main actions:
   - Member CRUD: `create_member/1`, `list_members/0`, `update_member/2`, `destroy_member/1`
   - Custom field value management: `create_custom_field_value/1`, `list_custom_field_values/0`, etc.
   - Custom field management: `create_custom_field/1`, `list_custom_fields/0`, etc.
+  - Settings management: `get_settings/0`, `update_settings/2`
 
   ## Admin Interface
   The domain is configured with AshAdmin for management UI.
@@ -45,5 +47,80 @@ defmodule Mv.Membership do
       define :destroy_custom_field, action: :destroy_with_values
       define :prepare_custom_field_deletion, action: :prepare_deletion, args: [:id]
     end
+
+    resource Mv.Membership.Setting do
+      # Note: create action exists but is not exposed via code interface
+      # It's only used internally as fallback in get_settings/0
+      # Settings should be created via seed script
+      define :update_settings, action: :update
+    end
+  end
+
+  # Singleton pattern: Get the single settings record
+  @doc """
+  Gets the global settings.
+
+  Settings should normally be created via the seed script (`priv/repo/seeds.exs`).
+  If no settings exist, this function will create them as a fallback using the
+  `ASSOCIATION_NAME` environment variable or "Mitgliederverwaltung" as default.
+
+  ## Returns
+
+    - `{:ok, settings}` - The settings record
+    - `{:ok, nil}` - No settings exist (should not happen if seeds were run)
+    - `{:error, error}` - Error reading settings
+
+  ## Examples
+
+      iex> {:ok, settings} = Mv.Membership.get_settings()
+      iex> settings.club_name
+      "My Club"
+
+  """
+  def get_settings do
+    # Try to get the first (and only) settings record
+    case Ash.read_one(Mv.Membership.Setting, domain: __MODULE__) do
+      {:ok, nil} ->
+        # No settings exist - create as fallback (should normally be created via seed script)
+        default_club_name = System.get_env("ASSOCIATION_NAME") || "Mitgliederverwaltung"
+
+        Mv.Membership.Setting
+        |> Ash.Changeset.for_create(:create, %{club_name: default_club_name})
+        |> Ash.create!(domain: __MODULE__)
+        |> then(fn settings -> {:ok, settings} end)
+
+      {:ok, settings} ->
+        {:ok, settings}
+
+      {:error, error} ->
+        {:error, error}
+    end
+  end
+
+  @doc """
+  Updates the global settings.
+
+  ## Parameters
+
+    - `settings` - The settings record to update
+    - `attrs` - A map of attributes to update (e.g., `%{club_name: "New Name"}`)
+
+  ## Returns
+
+    - `{:ok, updated_settings}` - Successfully updated settings
+    - `{:error, error}` - Validation or update error
+
+  ## Examples
+
+      iex> {:ok, settings} = Mv.Membership.get_settings()
+      iex> {:ok, updated} = Mv.Membership.update_settings(settings, %{club_name: "New Club"})
+      iex> updated.club_name
+      "New Club"
+
+  """
+  def update_settings(settings, attrs) do
+    settings
+    |> Ash.Changeset.for_update(:update, attrs)
+    |> Ash.update(domain: __MODULE__)
   end
 end