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/compose.yaml

@@ -20,11 +20,13 @@ x-library-ro: &library-ro
 # Локальная музыка (источники type=local): каталог хоста, например смонтированный диск /mnt/music.
 # Всем сервисам — только чтение; на запись (импорт загрузок) его видит только storage-agent — /library-host-rw
 # (docs/library-and-storage.md#запись-в-хранилище).
+# rslave: диск, смонтированный на хосте уже после старта контейнеров, становится виден внутри без перезапуска.
 x-library-host-ro: &library-host-ro
   type: bind
   source: ${LIBRARY_HOST:-${PIPU_DATA:-/srv/pipu}/library-host}
   target: /library-host
   read_only: true
+  bind: { propagation: rslave }
 
 x-restart: &restart
   restart: unless-stopped
@@ -96,6 +98,7 @@ services:
       - type: bind
         source: ${LIBRARY_HOST:-${PIPU_DATA:-/srv/pipu}/library-host}
         target: /library-host-rw
+        bind: { propagation: rslave }
       - ${PIPU_DATA:-/srv/pipu}/uploads:/uploads:ro
 
   recommender: