Fix multiple today fixtures creation and reset fixtures foreign key error

- Fix _get_or_create_today_fixture() to use created_at as fallback when start_time is NULL
- Replace _initialize_new_fixture() calls with _get_or_create_today_fixture() in _handle_start_game()
- Fix reset_fixtures() to use raw SQL DELETE statements for proper MySQL foreign key handling
- Delete tables in correct order: extraction_stats, bets_details, bets, match_outcomes, matches, match_outcomes_templates, matches_templates