docs(install): music on a local disk at /mnt/music, repo in /opt/pipu-music

Add an install step for mounting the music disk via fstab and wire it into
the stack: LIBRARY_HOST in .env points at the host folder, compose mounts it
as /library-host (read-only for api, nginx and analyzer; writable only for
storage-agent), nginx serves it through X-Accel, and install.sh validates the
path and records it in .env.

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

deploy/nginx/default.conf.template

@@ -48,6 +48,7 @@ server {
 
     # Аудио и обложки отдаёт nginx по заголовку X-Accel-Redirect от api (docs/playback.md).
     location /_internal/library/ { internal; alias /library/;          sendfile on; aio threads; }
+    location /_internal/library-host/ { internal; alias /library-host/; sendfile on; aio threads; }
     location /_internal/cache/   { internal; alias /cache/transcode/;  sendfile on; }
     location /_internal/covers/  { internal; alias /cache/covers/;     expires 30d; }
     location /_internal/uploads/ { internal; alias /uploads/;          sendfile on; }