feat(import): assign groups and fee types to imported members, creating missing groups

lib/mv/membership/import/header_mapper.ex

@@ -29,12 +29,21 @@ defmodule Mv.Membership.Import.HeaderMapper do
 
   Supports English and German header variants (e.g. "Email" / "E-Mail", "Join Date" / "Beitrittsdatum").
 
+  ## Special columns
+
+  - **groups** – Many-to-many relationship (through member_groups). Recognized via the
+    `groups_column_index` key (headers `Groups`, `Gruppen`, `Gruppe`). Comma-separated
+    names are resolved during processing; missing groups are auto-created.
+  - **membership_fee_type** – Recognized via the `fee_type_column_index` key (headers
+    `Fee Type`, `fee_type`, `membership_fee_type`, `Beitragsart`). Names are matched to
+    existing fee types; unknown names fall back to the default fee type.
+
   ## Fields not supported for import
 
   - **membership_fee_status** – Computed (calculation from membership fee cycles). Not stored;
-    cannot be set via CSV. Export can include it.
-  - **groups** – Many-to-many relationship (through member_groups). Import would require
-    resolving group names/slugs to IDs and creating associations; not in current import scope.
+    cannot be set via CSV. Export can include it. Fee-status header variants
+    (`Membership Fee Status`, `Bezahlstatus`, `Mitgliedsbeitragsstatus`) are explicitly
+    placed in the `ignored` list and never mapped.
 
   ## Custom Field Detection