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>

services/api/internal/dbtest/integration_test.go

@@ -103,7 +103,8 @@ func TestLibraryAndUploads(t *testing.T) {
 	assert.True(t, src.AcceptsUploads)
 	require.NotNil(t, src.WriteTemplate)
 
-	scanner := library.NewScanner(lib, fakeProber{}, library.NewProgressBoard(), log)
+	board := library.NewProgressBoard()
+	scanner := library.NewScanner(lib, fakeProber{}, board, log)
 	var fileIDs []uuid.UUID
 	for _, rel := range []string{"Кино/Группа крови/Кино - Группа крови.flac", "Кино/Группа крови/Кино - Закрой за мной дверь.flac"} {
 		id, err := scanner.ScanFile(ctx, src, root, rel)
@@ -114,6 +115,15 @@ func TestLibraryAndUploads(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, fileIDs[0], again, "rescan keeps the file row")
 
+	// Пустой каталог при известных файлах (диск не смонтирован) — скан прерывается и ничего не помечает пропавшим.
+	require.NoError(t, scanner.Start(src, t.TempDir(), false))
+	require.Eventually(t, func() bool { p, ok := board.Get(src.ID); return ok && !p.Running }, 10*time.Second, 50*time.Millisecond)
+	p, _ := board.Get(src.ID)
+	assert.Contains(t, p.Error, "is the disk mounted")
+	var missing int
+	require.NoError(t, pool.QueryRow(ctx, `SELECT count(*) FROM media_files WHERE source_id = $1 AND missing_since IS NOT NULL`, src.ID).Scan(&missing))
+	assert.Zero(t, missing)
+
 	// --- Отпечатки ---
 	todo, err := lib.FilesToFingerprint(ctx, true, uuid.Nil, 10)
 	require.NoError(t, err)