Stream activation

Control Room UI → Activate link on MAIN slot
       ↓
Backend updates DB + WS broadcast
       ↓
All iPads receive the new URL and start the HLS stream

Remote stream reset

Control Room UI → Restart (on a specific device)
       ↓
Backend sends device:reset via WS
       ↓
iPad saves last valid URLs → stops streams → 500ms → restores them

Link ping

Control Room UI → 📡 (on a specific link)
       ↓
Backend POST /api/links/ping → HTTP HEAD to the URL
       ↓
Control Room shows ✓ 342ms or ✗ ERR with error detail