Make minimal access and users without memberships non-billable

Resolves Make `minimal access users` or `users not assig... (#330663 - closed).

What does this MR do and why?

This MR makes minimal access users and users without group/project memberships non-billable on Premium self-managed.

Currently on Premium SM, these users consume license seats which creates a barrier for SSO and Restricted Access adoption and causes customers to pay for seats that provide no value. More details in #330663 (closed).

NOTE: Ultimate and GitLab.com already work correctly and are not in the scope of this change.

References

• Make `minimal access users` or `users not assig... (#330663 - closed)

Database review

 Nested Loop  (cost=1428653.67..3107524.94 rows=846470 width=1509) (actual time=28415.319..264349.218 rows=10424572 loops=1)
   Buffers: shared hit=60956387 read=14379658 dirtied=12
   WAL: records=13 fpi=12 bytes=92437
   I/O Timings: read=141853.663 write=0.000
   ->  HashAggregate  (cost=1428653.23..1438837.48 rows=1018425 width=4) (actual time=28415.093..41830.451 rows=12279298 loops=1)
         Group Key: members.user_id
         Buffers: shared hit=25843194 read=375197
         I/O Timings: read=3926.800 write=0.000
         ->  Index Only Scan using index_members_on_user_id_and_access_level_requested_at_is_null on public.members  (cost=0.56..1283893.81 rows=57903766 width=4) (actual time=0.215..15028.866 rows=57376816 loops=1)
               Index Cond: (members.access_level > 5)
               Heap Fetches: 2319975
               Buffers: shared hit=25843194 read=375197
               I/O Timings: read=3926.800 write=0.000
   ->  Index Scan using users_pkey on public.users  (cost=0.44..1.69 rows=1 width=1509) (actual time=0.018..0.018 rows=1 loops=12279298)
         Index Cond: (users.id = members.user_id)
         Filter: (((users.state)::text = 'active'::text) AND (users.user_type = ANY ('{0,6,4,13}'::integer[])) AND (users.user_type = ANY ('{0,4,5,15,17}'::integer[])))
         Rows Removed by Filter: 0
         Buffers: shared hit=35113174 read=14004461 dirtied=12
         WAL: records=13 fpi=12 bytes=92437
         I/O Timings: read=137926.863 write=0.000
Settings: seq_page_cost = '4', work_mem = '100MB', effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5'

Screenshots or screen recordings

Before	After

How to set up and validate locally

Setup GDK in self-managed mode with a Premium license.

1. Create a test user and do not assign any projects or groups to it
2. Create a test user on the UI and it to a group with access level Minimal Access
3. On the console, find the test users and verify billable status:

no_membership_user = # Find user created above by ID
minimal_user = # Find user created above by ID

User.billable.include?(minimal_user)      # Should be false
User.billable.include?(no_membership_user) # Should be false

minimal_user.using_license_seat?      # Should be false
no_membership_user.using_license_seat? # Should be false

3. Check Admin UI:
   • Go to Admin Area → Users
   • Verify minimal access user has no "Is using seat" badge
   • Click on user → Verify "Is using license seat: No"

MR acceptance checklist

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