Get the active playlist and next re-evaluation deadline

Active assets + next deadline, evaluated against server time.

Intended for the C++ viewer (GH #2906 Phase 3) so the viewer no longer needs Django ORM access or its own Asset.is_active() re-implementation. The Python viewer keeps using anthias_viewer.scheduling.generate_asset_list() directly until Phase 3 swaps it.

Internal-auth gated for the same reason as AssetRecheckViewV2: the viewer can't attach operator BasicAuth, so it presents the shared token derived from anthias.conf.