Improves model spec performance by additional ~2% (14.5s → 14.1s)
Combined with chapter optimization: 17.0s → 14.1s (~17% total improvement)

The default :event fabricator was creating a sponsorship for every
event, but most tests don't need this. Removed the after_build
callback from the default fabricator and added :event_with_sponsorship
for tests that need it.

Includes test fixes for tests broken by this change:
- event_spec.rb: Use :chapter instead of :chapter_with_groups
- manage_event_spec.rb: Use :chapter instead of :chapter_with_groups
- filtering_sponsors_list_spec.rb: Use :sponsor instead of :sponsor_with_contacts
- sponsor_spec.rb: Use :sponsor instead of :sponsor_with_contacts

Improves model spec performance by additional ~2% (14.1s → 13.8s)
Combined optimizations: 17.0s → 13.8s (~19% total improvement)

The students and coaches fabricators were creating 5 members each,
but tests only need 2 members per group. Reduced member count to
reduce object creation overhead.

Includes test fix for meeting_spec.rb banned members test:
- Updated to work with 4 total members (2 students + 2 coaches) instead of 10