test: add integration tests for vendor orders API and background tasks
Add comprehensive integration tests for:
- Vendor orders API (list, detail, status updates, pagination, filtering)
- Letzshop historical import background task (success, errors, progress)
- Subscription background tasks (period reset, trial expiration, Stripe sync)
These 39 new tests improve coverage for background task functionality
and vendor order management.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Samir Boulahtit
23a5ccbcff