Batch user role loading

[slick-daddy]

The admin users list requests roles for every user. Previously, User.Roles executed one database query per user, creating an N+1 query pattern.

This change adds a UserRolesByID dataloader that:

• Uses the existing StringsLoader because roles are stored as strings and converted to GraphQL enums by the resolver.
• Batches up to 1,000 user IDs per fetch, matching existing dataloader limits.
• Waits 1 millisecond to collect role requests from multiple users into one query.
• Delegates database access to User.LoadRoles, keeping SQL and service logic outside the resolver.
• Maps results back to the original user-ID order, including users with no roles.