Add migration to backfill user_group_member_roles for direct membership

What does this MR do and why?

• Resolves part-1 of #520189 (closed)
• Creates a background migration to populate the user_group_member_roles table from the members table for group members that have a member role assigned to them.
• As part of that, it creates a temporary index on the members table.
• This MR is part of a bigger epic to cache member roles, #513033

Database

1. Migration:

main: == 20250804025048 AddTmpIndexToMembersForGroupMembersWithMemberRole: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0107s
main: -- index_exists?(:members, [:id, :source_id], {:name=>:tmp_idx_members_on_id_for_group_members_with_member_role, :where=>"member_role_id IS NOT NULL AND source_type = 'Namespace'", :algorithm=>:concurrently})
main:    -> 0.0061s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:members, [:id, :source_id], {:name=>:tmp_idx_members_on_id_for_group_members_with_member_role, :where=>"member_role_id IS NOT NULL AND source_type = 'Namespace'", :algorithm=>:concurrently})
main:    -> 0.0065s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20250804025048 AddTmpIndexToMembersForGroupMembersWithMemberRole: migrated (0.0386s) 

2. BBM Query plan

MR acceptance checklist

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Related to #520189 (closed)