test(backend): Expand test coverage for commands, auth, services, and tasks

[marevol]

Summary

• Add comprehensive backend test coverage across management commands, authentication, services, views, serializers, and Celery tasks
• 11 files changed with +1,360 lines of new/updated tests

Changes Made

New test modules

• test_assign_owners_command.py — Tests for assign_owners management command (assign, dry-run, invalid user, no unowned records)
• test_create_api_key_command.py — Tests for create_api_key command (create, invalid project/owner/scope, duplicate name, expiry)
• test_create_test_users_command.py — Tests for create_test_users command (create, update, invalid format)
• test_project_service.py — Tests for project service (create, duplicate name, cross-user isolation, get/delete owned, summary)
• test_user_service.py — Tests for user service (list, create, change password, toggle active, delete)

Extended existing tests

• test_authentication.py — RequireManagementScope permission (read/write/JWT), ambiguous API key prefix handling
• test_data_post.py — Training data preview endpoint (returns data, n_rows param, unauthenticated 401, cross-owner denial)
• test_event_views.py — Conversion event access control (API key wrong project, JWT non-owner, slot/project mismatch, API key auth for events)
• test_serializers.py — Retraining schedule and deployment slot serializer validation
• test_schedule_service.py — Added locmem cache fixture for Redis-free testing
• test_tasks.py — Celery tasks for training, tuning, scheduled retraining, error/edge cases (missing data, cancelled jobs, unknown recommender)

Testing

• All tests use pytest-django with @pytest.mark.django_db
• In-memory cache (locmem) used where Redis is not available
• Tests cover both happy paths and error/edge cases

Additional Notes

• autouse locmem cache fixtures added to test modules that interact with Django cache (avoids Redis dependency in test runner)