iOS downloads: background sessions, queues, and a real state machine

Overhauled the download infrastructure with background URLSessions, a download queue, and a proper state machine. Models are big — the 12B Dense is 6.1 GB. You can't just fire a URLSession.shared download and pray. Background sessions survive app suspension, the queue manages concurrent downloads without thrashing bandwidth, and the state machine handles queued/downloading/paused/completed/failed transitions cleanly. Also added an AppDelegate for background download session events and iOS push notifications when downloads complete. The mobile model hub now shows queued and paused states properly — small detail, but it's the difference between "is this thing still downloading?" and knowing exactly where you stand.