Optimize or replace Admin API pagination

[VaibhavAcharya]

The Admin API user listing (FindUsersInAudience in models/user.go) uses Pop's built-in Paginate() which does SELECT COUNT(*) + SELECT ... LIMIT OFFSET. This doesn't scale well for large user tables.

Related concerns:

• The COUNT(*) query is expensive on large tables
• MySQL may choose suboptimal indexes for different instance_id values — index hints are not supported through Pop's query builder
• The filter query uses LIKE with wildcards on a JSON column with COLLATE, which can't use indexes effectively
• Consider cursor-based pagination or removing pagination entirely if clients don't need total counts

Moved from CI 25, 26, 27