feat: storage writes via storage-agent, accepts_uploads and write_template (M5.5)

- storage-agent: POST /internal/files copies a staged file into a source
  (temp file, fsync, SHA-256 check, hard link so nothing is overwritten,
  " (2)" suffix on conflict, owner PIPU_FILE_UID/GID) and
  POST /internal/files/test. NFS sources that accept uploads are mounted
  rw in an agent-only /library-rw and bind-mounted read-only for others;
  local sources are written through /library-host-rw.
- meta: write templates ({track:02}, {disc:1-}, [optional]) with name
  sanitising and length limits, suggestion from the source's regexes
  and a reversibility check against 50 catalogue tracks.
- API and admin UI: accepts_uploads / write_template on sources,
  write-template suggest and check endpoints, write test result.

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

deploy/compose.yaml

@@ -18,8 +18,8 @@ x-library-ro: &library-ro
   bind: { propagation: rslave }
 
 # Локальная музыка (источники type=local): каталог хоста, например смонтированный диск /mnt/music.
-# Всем сервисам, включая storage-agent, — только чтение: импорт загрузок ещё не реализован,
-# запись откроем вместе с ним (docs/library-and-storage.md).
+# Всем сервисам — только чтение; на запись (импорт загрузок) его видит только storage-agent — /library-host-rw
+# (docs/library-and-storage.md#запись-в-хранилище).
 x-library-host-ro: &library-host-ro
   type: bind
   source: ${LIBRARY_HOST:-${PIPU_DATA:-/srv/pipu}/library-host}
@@ -80,6 +80,12 @@ services:
     environment:
       INTERNAL_TOKEN: ${INTERNAL_TOKEN:?заполните INTERNAL_TOKEN в .env (sudo ./install.sh)}
       LIBRARY_ROOT: /library
+      # rw-точки NFS-хранилищ с приёмом загрузок — каталог внутри контейнера, наружу не виден.
+      LIBRARY_RW_ROOT: /library-rw
+      LIBRARY_HOST_RW_ROOT: /library-host-rw
+      UPLOADS_ROOT: /uploads
+      PIPU_FILE_UID: ${PIPU_FILE_UID:--1}
+      PIPU_FILE_GID: ${PIPU_FILE_GID:--1}
       LOG_LEVEL: ${LOG_LEVEL:-info}
     volumes:
       - type: bind
@@ -87,6 +93,9 @@ services:
         target: /library
         bind: { propagation: rshared }
       - *library-host-ro
+      - type: bind
+        source: ${LIBRARY_HOST:-${PIPU_DATA:-/srv/pipu}/library-host}
+        target: /library-host-rw
       - ${PIPU_DATA:-/srv/pipu}/uploads:/uploads:ro
 
   recommender: