Add pagination to GET /api/v1/users admin endpoint #119

The GET /api/v1/users endpoint currently loads the entire Users table plus all StorageConfigs in a single query via GetAllAsync. This is acceptable for early development but will become a problem at scale.

Before this endpoint ships to a multi-user deployment, it should support cursor- or page-based pagination with a configurable page size, and GetAllAsync in IUserRepository / EfUserRepository should accept pagination parameters accordingly.