The scheduler.py already supported "daily HH:MM,HH:MM,HH:MM" format
(commit 5f27d05), but config.py validation regex only accepted single
time "daily HH:MM", causing data-refresh to crash on startup.
Also adds:
- tests/test_config_sync_schedule.py (16 test cases)
- Makefile with validate-config target for CI/CD integration
