fix(auth): prevent unexpected logout during file upload (#48)

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

Add new test modules for management commands (assign_owners, create_api_key,
create_test_users), project service, and user service. Extend existing tests
for authentication (scope permissions, ambiguous prefix), data upload
(preview endpoint, cross-owner access), event views (API key access control,
slot/project mismatch), serializers, schedule service, and Celery tasks
(training, tuning, scheduled retraining, error handling).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(auth): prevent logout during file upload by refreshing token proactively

- Add ensureFreshToken() to auth store: checks token expiry and refreshes
  if within 60 seconds of expiry, preventing stale-token upload failures
- Add visibilitychange listener: re-schedules proactive refresh when the
  tab regains focus, covering tokens that expired in background tabs
- Add 401 retry in DataUploadPage: if the XHR returns 401 (token expired
  mid-upload), refresh the token and retry the upload once automatically
- Attach .status to XHR errors so the 401 can be detected in catch blocks
- Update tests: switch nextTick → flushPromises for async ensureFreshToken,
  add 23 new tests covering all new behaviours

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* style(tests): apply ruff formatting to test files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: marevol