feat(vereinfacht): add DB schema, config and setting attributes

- Migrations: vereinfacht_contact_id on members, vereinfacht_* on settings
- Mv.Config: Vereinfacht ENV/Settings helpers, vereinfacht_configured?, contact_view_url
- Setting: vereinfacht_api_url, api_key, club_id

lib/membership/setting.ex

@@ -69,7 +69,10 @@ defmodule Mv.Membership.Setting do
         :club_name,
         :member_field_visibility,
         :include_joining_cycle,
-        :default_membership_fee_type_id
+        :default_membership_fee_type_id,
+        :vereinfacht_api_url,
+        :vereinfacht_api_key,
+        :vereinfacht_club_id
       ]
     end
 
@@ -81,7 +84,10 @@ defmodule Mv.Membership.Setting do
         :club_name,
         :member_field_visibility,
         :include_joining_cycle,
-        :default_membership_fee_type_id
+        :default_membership_fee_type_id,
+        :vereinfacht_api_url,
+        :vereinfacht_api_key,
+        :vereinfacht_club_id
       ]
     end
 
@@ -225,6 +231,26 @@ defmodule Mv.Membership.Setting do
       description "Default membership fee type ID for new members"
     end
 
+    # Vereinfacht accounting software integration (can be overridden by ENV)
+    attribute :vereinfacht_api_url, :string do
+      allow_nil? true
+      public? true
+      description "Vereinfacht API base URL (e.g. https://api.verein.visuel.dev/api/v1)"
+    end
+
+    attribute :vereinfacht_api_key, :string do
+      allow_nil? true
+      public? true
+      description "Vereinfacht API key (Bearer token)"
+      sensitive? true
+    end
+
+    attribute :vereinfacht_club_id, :string do
+      allow_nil? true
+      public? true
+      description "Vereinfacht club ID for multi-tenancy"
+    end
+
     timestamps()
   end