Complete people-domain move into members app; keep names on Member

Finish relocating Member/Family/FamilyMembership from authentication into
a dedicated members app, and revert the half-applied move of member names
onto the global User.

- members: add first_name/last_name back to Member (the whole codebase —
  tests, CSV importer, club app, admin — assumes them, and login-less
  children in families need a name); restore local ordering/index and the
  member__last_name lookups in Family.__str__ and FamilyMembership.
- authentication: drop first_name/last_name from User; get_full_name/
  get_short_name delegate to the linked member, else fall back to email.
- migrations: create members.0001_initial, repoint club.ClubMembership FK
  (club.0005), delete the models from authentication (0004, rewritten to
  plain DeleteModel ops in child-first order to avoid a SQLite table-remake
  crash).
- fix stale imports across apps (authentication/club tests, CSV importer)
  and missing imports/URLs in members tests; add missing _ import in
  members/admin.py; export MemberCsvImporter from members.services.

Full suite green (64 tests), ruff clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

authentication/migrations/0004_remove_familymembership_family_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.6 on 2026-07-11 22:09
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('authentication', '0003_alter_family_name_alter_familymembership_family_and_more'),
+        ('club', '0005_alter_clubmembership_member'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='FamilyMembership',
+        ),
+        migrations.DeleteModel(
+            name='Family',
+        ),
+        migrations.DeleteModel(
+            name='Member',
+        ),
+    ]