Detect a disconnected or read-only recordings drive and surface it
without stopping recording or live view:
- storage.go: probe path existence, mount point, writability and free
space every 30s; capture mount expectation at startup so a clean
unmount (writes falling through to root fs) is also caught
- GET /api/storage endpoint; storage fields in /api/status
- red UI banner while storage is unavailable
- log ALERT on loss, OK on recovery
