fix(library): survive a music disk that is not mounted

On the test server the host music folder was not mounted, so every stream and
fingerprint failed with no such file, and a scan of the empty folder would have
marked the whole library missing and purged it after 30 days.

- a scan of an empty source folder with known files aborts with a clear error
- fingerprinting skips such a source with one warning instead of one per file
- LIBRARY_HOST mounts use rslave, so a disk mounted after start shows up
- update.sh refuses to deploy with an empty LIBRARY_HOST

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/update.sh

@@ -26,6 +26,15 @@ for key in JWT_PRIVATE_KEY STREAM_SIGNING_KEY; do
   fi
 done
 
+# Папка с музыкой пуста — почти всегда это несмонтированный диск: api отдавал бы ошибки воспроизведения.
+LIBRARY_HOST="$(sed -n 's/^LIBRARY_HOST=//p' .env)"
+if [ -n "$LIBRARY_HOST" ] && [ -z "$(ls -A "$LIBRARY_HOST" 2>/dev/null)" ] && [ "${PIPU_ALLOW_EMPTY_LIBRARY:-}" != 1 ]; then
+  echo "LIBRARY_HOST=$LIBRARY_HOST пуст или не существует — диск с музыкой не смонтирован?" >&2
+  echo "  проверьте: findmnt $LIBRARY_HOST; смонтируйте (sudo mount -a) и повторите." >&2
+  echo "  Если папка и должна быть пустой: PIPU_ALLOW_EMPTY_LIBRARY=1 ./update.sh ${1:-}" >&2
+  exit 1
+fi
+
 REF="${1:-}"
 git fetch --tags --prune --quiet origin