Optimize group fabricator: Reduce member count 5→2

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.

Author: mroderick

spec/fabricators/group_fabricator.rb

@@ -6,10 +6,10 @@
 
 Fabricator(:students, from: :group) do
   name 'Students'
-  members(count: 5)
+  members(count: 2)
 end
 
 Fabricator(:coaches, from: :group) do
   name 'Coaches'
-  members(count: 5)
+  members(count: 2)
 end