feat(api): pagination on admin list endpoints

[trinhvandat]

Context (P1 — scale)

No Pageable usage exists in main source — all admin list endpoints return unbounded collections (orgs, projects, environments, flags, members).

Scope

• Spring Data Pageable on repository/service/controller list paths with sane defaults (size 20, max 100)
• Consistent paginated response envelope (content, page, size, totalElements, totalPages) — align with the API response format convention
• OpenAPI docs reflect the parameters
• SDK evaluation endpoint explicitly excluded (SDKs need the full set)

Acceptance criteria

• All admin list endpoints accept page/size/sort with enforced max page size
• Response envelope consistent across endpoints; existing consumers/tests updated
• Default sort is deterministic (e.g. createdAt,id) — no flaky ordering