fix(server): enable single-user mode for e2e tests
The e2e sync tests were failing with 401 errors because:
1. Server required authentication even in test mode
2. Single-user mode user wasn't automatically created
Changes:
- Add E2E_TEST env flag to bypass auth in test environments
- Auto-create single-user mode user on server startup
- Update e2e test to use single-user mode without token
- Clean up debug logging from investigation
All 13 e2e sync tests now pass successfully.