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>

manage.py

@@ -1,22 +1,19 @@
 #!/usr/bin/env python
 """Django's command-line utility for administrative tasks."""
+
 import os
 import sys
 
 
 def main():
     """Run administrative tasks."""
-    os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'clubmanager.settings')
+    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "clubmanager.settings")
     try:
         from django.core.management import execute_from_command_line
     except ImportError as exc:
-        raise ImportError(
-            "Couldn't import Django. Are you sure it's installed and "
-            "available on your PYTHONPATH environment variable? Did you "
-            "forget to activate a virtual environment?"
-        ) from exc
+        raise ImportError("Couldn't import Django. Are you sure it's installed and available on your PYTHONPATH environment variable? Did you forget to activate a virtual environment?") from exc
     execute_from_command_line(sys.argv)
 
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     main()